I've worked in education previously. Apart from being mandated by the gov't, lots of kids have issues so making allowances for them makes sense. We had to retrofit some of the WCAG functionality into our product. Possible but better to have it as a requirement up front. The thing is, modern browsers are really good about accessibility so you gotta go out of your way to mess it up. In my opinion it's like having ramps…
As an example, if you make mouse-based users do something in 2 clicks, but keyboard-based users tab through 100 elements to do the same thing, that is going to be flagged as not accessible even if it does pass WCAG validators.
As far as startup MVPs go, I agree completely that it isn't required on day one, but the design should take it into consideration to be ready for the future.