I may just be really tired and not understanding, but I think we're talking past each other about two different things.
"Solo founder bootstrapped company" sounds to me like "90% of small businesses", and I'm pretty sure virtually none of them have founder vesting, and yet they manage to hire people without the business owner having to vest their shares. Do you just mean in the case where they're going to issue shares to employees? So if I've been running this company for a couple years and I hire someone that I want to compensate with equity (which would be pretty unlikely, given that I think that's generally a crummy deal for both of us), why exactly can't I set them up on a normal vesting schedule?
Similarly, I suspect VCs are perfectly willing to invest in teams where the sole founder didn't previously have vesting setup. Indeed, I'd expect that's nearly always the situation.
So if I want to raise money, why can't we work out a vesting schedule at that point, which I suspect is how it goes 99% of the time that investors want solo founders to setup vesting. They weren't vesting at all before then, because, why would they? Doesn't make any sense.
This seems a bit like buying life insurance when you have no dependents, because you might get married and have kids later. Yeah, so get the life insurance then. In the meantime, who is getting that money if you die?