Lineages seem to be the downfall of empires. I wonder if the Roman Empire would have survived longer had Marcus Aurelius (or anyone after him) not chosen their heir by biology, but by virtue, as the previous emperors did.

A large proportion of Roman emperors became heirs via adoption for this purpose, not via actual inheritance.

Marcus had many children, many of whom died early. He was succeeded by his son Commodus, who's depiction in Gladiator is well known (and invariably inaccurate).

The depiction of Commodus by historians was not favorable either. He's widely thought of as the beginning of the end of the Roman Empire.

It would have probably caused a civil war itself. Commodus was the accepted heir, I doubt Marcus Aurelius could have chosen anyone else as long as Commodus was alive. All of the previous "good" emperors had no male heirs to pass the empire on to.

The point of heir by birth is to eliminate questions of who will be heir and the power struggles. You can raise the heir and any backups with that role in mind from day 1 so they're amenable to it and you also have time to figure out if they're idiots or not and can assemble other governance structures (e.g. delegate specific responsibility into more merit-based advisor roles to keep your heir from screwing it up) to insulate against that if needed. The problem is when you have a bunch of idiots in a row or when you have a bunch of good kings in a row (and all the surrounding official positions get full of idiots). Generally speaking, of course.