Developers spend far too much time thinking up justifications not to build accessible software. 99.9999% of the time no such user as Chris exists, so the argument is nonsense - if you build an accessible app and you don't put things in the way of users modifying it using their accessibility software, then it will work better for everyone. Users with accessibility needs don't want special, complex additions. They want software that works, just like everyone else.
If you really can point to a specific user and define 3 months of work for that person to use the app then you're lucky. You really know your users, and you can determine exactly how much of a problem not supporting that user is. That's very unusual.
The point here isn't that people have unique disabilities though, it's the assertion that people have unique disabilities that require unique solutions. That is entirely false. If you have a single blind user you don't have to code "special stuff for blind people"; you just have to make an accessible app and their screenreader will do all the "special stuff for blind people" work. The same is true for pretty much every disability. Just write code that has the necessary accessible structure and tags and let the user do turn it in to an app that works for them.
A key thing to consider is that a user with accessibility issues wants a solution that works on every website and every app. If you're thinking you need to spend months of time coding accessibility features just for your app, and expand that out to every other app, then clearly something is very wrong. That's why we have standards and best practises; if you follow them then your app will 'just work' with accessibility software.
And, as a very useful bonus feature, if you've written your app with semantic markup, a good information hierarchy, a contrast-y color scheme, and and a bit of thought then it'll also work better for every single other user regardless of their accessibility needs. It'll simply be a better app.
If you really can point to a specific user and define 3 months of work for that person to use the app then you're lucky. You really know your users, and you can determine exactly how much of a problem not supporting that user is. That's very unusual.