While proof-of-payment certainly has its benefits, I believe that's part of my question. GLX essentially uses a "proof-of-payment" system today, just a terribly executed one.
The GLX is already designed to reap most benefits of proof-of-payment systems, most notably all-door boarding. If they're indeed beneficial for GLX, the answer to my question should be a resounding no. But that would require the (economic and social) benefits of not having riders go through fare gates to outweigh harms of fare evasion, and that seems hard to analyze.
In an AFC 2.0 world, I imagine the main difference will be that there will be more visible tap-to-pay card readers at station entrances, instead of CharlieCard machines for fare validation. They'll certainly serve as a reminder for riders to pay, but the question is whether they'll actually tap. At least with the current implementation, many or most riders don't, based on the observations of myself and others here. And once riders develop the habit of evading fares and knowing they'll face no consequences, it can be hard to change.
I do think AFC 2.0 will be crucial for Green Line surface stations and buses, but GLX seems like a different case (edit: at least if patrolling is as non-existent as today).