I feel like issues like this would "work themselves out" in the hypothetical scenario, as the saying goes. That is, it's not to say ECNL (and other leagues) would necessarily revert to BY, but rather that they would each decide how significant of a factor SY actually is (in terms of the various arguments put forth, such as playing with classmates, reducing "trapped player" instances, etc.). If those factor were more important to the parents than, for example, playing in specific tournaments (and assuming those tournaments didn't make allowances for SY based teams, although they probably would), then they would stick to SY. OTOH, if the other concerns were actually more important, they could switch back to BY, based on what their customers preferred.
Meanwhile, the rest of us could continue to play with our existing teams in the existing leagues, and nobody else would need to pay much attention to that drama. I don't personally care, for example; my kid is not going pro, and I just want the years of him playing soccer before going to college to be as enjoyable and drama/disruption free as possible.
If ECNL moves to SY, then all the ECNL teams/clubs will go SY, as most the matches they play are against other ECNL teams. Not doing SY, would put them at a disadvantage in league, showcase, and playoff instances. They will forego Surf Cup, and all the other national/local non-ECNL events. MLS Next won't bother going to those tournaments either as the only solid competition they will have is against other MLS Next clubs. That's something they get with MLS Next events (e.g., MLS Next Fest). I agree with @Carlsbad7 -- I think tournaments as we know them today will pewter out and more walled gardens will be formed. Personally I think either SY or BY needs to mandated. I don't really care which one, but this ambiguity is going to have a bigger impact than what most are thinking.