True Story: I worked for a while on a website where users would upload and share tutorials on various topics. As the company behind it grew, it had the genius idea of selling print copies of their best tutorials. At some point, we were looking for a Python tutorial to publish. We select our favorite one, get in touch with the author and were genuinely delighted to find out that he was blind, accessing our app via screen reader.
He went on to publish a book (last I heard selling at over 10 000 units in less than a year topping Amazon's sales in its category). It was our app (the general focus on accessibility, independance from Js was a big part of it) that allowed this cool blind kid to publish a best sellers book. The odds of something like this happening, clearly, aren't very high, but when they do it's instant gratification for the developer who worked on making it accessible.
If you're too hung up on numbers and ROI, think of all the benefits the company got from the positive exposure it gathered from the press around the incident. In any case, unless the core of your app is in JS, it's always a good idea to provide alternatives.