Antoninus might have been only intended as a stop gap until Marcus Aurelius was old enough, but he actually reigned for decades (so Marcus Aurelius was in his late 40s when becoming Emperor), and his reign is generally considered peak Roman Empire - not the largest border extension as with Trajan, but in terms of culture, law, peace within and mostly without. No personal or political scandals or drama, which is why you surely haven't heard of him. Idyllic years and a happy population do not headlines or sensational gossip make. Anyway, Antoninus eventually dies happy in the knowledge that the succession is arranged, and indeed Marcus Aurelius becomes Emperor. Marcus Aurelius is also the first Emperor since Vespasian to have a living biological son (he had two, actually, but one of them died as a child) at his disposal when it comes to succession arrangements. And his son is, drumroll, Joacquin Phoenix, I mean, Commodus. Which is why it's commonly held against Marcus Aurelius that after a series of adopting a competent guy to succeed you made the Empire florish, he went for the biological option instead (and he did make it clear in Commodus' teenage years that his son was to succeed him, by naming him Caesar and jointly ruling with him during his last years of life).
However, it's worth pointing out that a) none of his predecessors among the hallowed Five had a biological son to choose, and b), as Mike Duncan argues, what would have been the alternative? Because if, say, Marcus Aurelius had adopted someone else and made that guy Caesar instead, then once he died certainly any malcontent or enemy of the other chosen guy would have rallied behind Commodus, and there would have been civil war, the one thing any Roman Emperor should avoid setting up. Unless Marcus Aurelius would have had his son killed, and while of course in retrospect that would have been better for the Empire, he wasn't Constantine. He could not kill his own son.
So Commodus succeeds as planned, and promptly becomes one of the Worst Five Emperors Ever. Seriously. Even Nero and Caligula have their defenders who argue that their names got blackened by their successors, but Commodus was awful in every aspect, and that's why not just Gibbon sees the Decline and Fall of the (Western) Empire start with his reign. In stark contrast to, say, Domitian, who might have been an awful paranoid person but was a competent dictator working hard, Commodus leaves the actual governing to a series of favourites whom he kills off when he's sick of them and goes for non-stop games instead. Soon, corruption runs so rampant that in the year of Commodus' death, the office of consul - in the days of the Republic the highest office a man could hold, the climax of the cursus honorum, whom the years were counted after - is held by no less than 25 men, sold to everyone of them, there are also non-stop treason trials, and when Commodus (who of course doesn't bother arranging the succession, he's busy renaming Rome into Commodum instead) is finally put out of his and everyone else's misery, the most infamous scenes of all concerning the succesion in the Roman Empire happen. Yes, it's the year of the Five Emperors now (inflation is a thing).
Prelude, directly after Commodus is killed: Pertinax (son of a freedman, Urban Prefect of Rome at the time) becomes Emperor. Pertinax actually wants to reform the rampant corruption and wants to start with the Praetorian Guard. The Praetorian Guard says "are you kidding?" and kills Pertinax. Then the Praetorian Guard literally auctions off the office of Emperor. (Pertinax having reigned a little more than 80 days at this point.) The last two bidders are Sulpicianus and Julianus. After hours of bidding, Sulpicianus promised 20,000 sesterces to every soldier; Marcus Didius Julianus, fearing that Sulpicianus would gain the throne, then offered 25,000 and for that sum buys the Roman Empire. For a mere 66 days, it turns out, because by then news have spread to the provinces, several generals, among them the eventual winner, Septimius Severus (first African Emperor of Rome), say ARE YOU KIDDING ME? and set their troops marching. Didius dies, murdered, with the line "“But what evil have I done? Whom have I killed?" He has a point. Anyway, from this point onwards, succession to the imperial throne is pretty much a matter of who has the strongest troops and/or murders his rivals first.
Imperial Succesions :II
However, it's worth pointing out that a) none of his predecessors among the hallowed Five had a biological son to choose, and b), as Mike Duncan argues, what would have been the alternative? Because if, say, Marcus Aurelius had adopted someone else and made that guy Caesar instead, then once he died certainly any malcontent or enemy of the other chosen guy would have rallied behind Commodus, and there would have been civil war, the one thing any Roman Emperor should avoid setting up. Unless Marcus Aurelius would have had his son killed, and while of course in retrospect that would have been better for the Empire, he wasn't Constantine. He could not kill his own son.
So Commodus succeeds as planned, and promptly becomes one of the Worst Five Emperors Ever. Seriously. Even Nero and Caligula have their defenders who argue that their names got blackened by their successors, but Commodus was awful in every aspect, and that's why not just Gibbon sees the Decline and Fall of the (Western) Empire start with his reign. In stark contrast to, say, Domitian, who might have been an awful paranoid person but was a competent dictator working hard, Commodus leaves the actual governing to a series of favourites whom he kills off when he's sick of them and goes for non-stop games instead. Soon, corruption runs so rampant that in the year of Commodus' death, the office of consul - in the days of the Republic the highest office a man could hold, the climax of the cursus honorum, whom the years were counted after - is held by no less than 25 men, sold to everyone of them, there are also non-stop treason trials, and when Commodus (who of course doesn't bother arranging the succession, he's busy renaming Rome into Commodum instead) is finally put out of his and everyone else's misery, the most infamous scenes of all concerning the succesion in the Roman Empire happen. Yes, it's the year of the Five Emperors now (inflation is a thing).
Prelude, directly after Commodus is killed: Pertinax (son of a freedman, Urban Prefect of Rome at the time) becomes Emperor. Pertinax actually wants to reform the rampant corruption and wants to start with the Praetorian Guard. The Praetorian Guard says "are you kidding?" and kills Pertinax. Then the Praetorian Guard literally auctions off the office of Emperor. (Pertinax having reigned a little more than 80 days at this point.) The last two bidders are Sulpicianus and Julianus. After hours of bidding, Sulpicianus promised 20,000 sesterces to every soldier; Marcus Didius Julianus, fearing that Sulpicianus would gain the throne, then offered 25,000 and for that sum buys the Roman Empire. For a mere 66 days, it turns out, because by then news have spread to the provinces, several generals, among them the eventual winner, Septimius Severus (first African Emperor of Rome), say ARE YOU KIDDING ME? and set their troops marching. Didius dies, murdered, with the line "“But what evil have I done? Whom have I killed?" He has a point. Anyway, from this point onwards, succession to the imperial throne is pretty much a matter of who has the strongest troops and/or murders his rivals first.