As I implied myself, there is no guarantee. Also, such a distinct change would need to be formulated, propagated and take root. [This did actually NOT occur.] The era of the adoptive emperors (in such a scenario at least 83 years, once Commodus is butterflied perhaps more) should be long enough.
Once that is done, the chances of ursupation fall markedly. Note that such crises usually fall into succession crises, with the exception of the mid-3rd century which saw a complete erosion of Imperial legitimacy.
And I disagree strongly with that (on the chances of usurpation). Let's say you have a century and a half of adoptive emperors who have been of at least reasonable ability and popularity - Constantine VII (post-classical Rome, I know, but he's a good example) will do.
Along comes someone like (Whether we count the "Byzantine" era as Roman, they're recognizable people) Michael VI, and the general Isaac Comnenus.
The army is going to have a lot more love for Isaac than Michael. What do?
Saying that the system prevents having anyone like Michael in the first place . . . ha ha.
The way I fleshed out the concept makes clear, that at any point of time (as was actually only the case during the end of Nerva's reign and during the reign of Antoninus Pius) a well-regarded man (enjoying, at least initially, the trust of the Emperor and of the Senate) would be the earmarked successor. He would be in a very strong and legitimate position. Also, by decisively denying the possibilities of the own-offspring, the position of adopted emperor becomes a (remote) career possibility for generals such as Traianus.
No more than the legitimacy of any
Porphyrogenitus.
This concept would also markedly lower the chances that the elites of the empire see themselves driven to violently depose an at least instable personality (Caligula, Nero, Domitian, Commodus); thus creating new crises of legitimacy.
How? I missed that part completely, I'm afraid.
Thus, POD 1 might be that Hadrian and Antoninus Pius not only work out the concept (which is close to what had been decided upon in 137/138), but also codify it as a central law of the principate, the LEX IMPERII OPTIMI.
POD 2 needs to be Marc Aurel adhering to this principle by never taking his own son into account, but constantly having a qualified and legitimate successor-in-waiting (as he was himself).
Laws are broken by usurpers all the time. Why is this going to be particularly effective at deterring them?
Frankly, adoptive emperors sounds like a weak method of avoiding poor emperors and utterly useless at avoiding usurpers.