You really should be building with a11y in mind from the get-go. It's 2019. The tooling is much, much better than it was in years past. It shouldn't be impossible for any enterprise to make a reasonably accessible application.

Nobody even expects 100% perfectly, semantically correct HTML documents. And it takes relatively little work to get reasonably good support. Like anything, the investment has diminishing returns but the 80/20 rule applies.