Oh, I love this excuse.
Our code base is too old to be accessible. We'd rather start a "new system" and do it right.
Yeah, right. If your house gets too moldy to live in, do you just sleep in a hotel until you tear it down and build a new one? I don't think so. You're not going to put up with the mold, the smell, the dark green shit on your ceiling for long. You'll spend the time and the money to go room to room and fix the underlying foundation. You'll sleep in the house until it's done. And it'll be done faster because you're feeling that pain every day.
Legacy code is unavoidable. What you write today is legacy tomorrow. You might as well get used to that reality and stop looking at legacy code as your get out of jail free card.
Your users don't care if your system is written in Angular 1, jQuery or Cobol. They just need it to fucking work, accessible and all. If your product is actively excluding people, it's broken, not "legacy." Deal with the mess.
The laws don't say "You are required to be accessible, unless your code base is legacy, then nevermind."
If you think you'll do it right in the new system, you're dreaming. Your "new system" will become legacy within a year. If you can't be bothered to fix your existing system now, what makes you think you'll magically care this time next year?
The habits you've built along the way to building your "legacy" system are hard to get rid of. You've ignored accessibility so far. That'll follow you into your "fresh start."
But I get it. Really I do.
You can't rebuild your whole code base in one go. And that includes accessibility.
But you can start.
You can start with fixing colour contrast, adding alt text, fixing keyboard navigation, a proper headings structure, form labels. You know, those basic WCAG Level A things that aren't even that complicated to get right.
"But it's too hard!" Like, I don't know, have you even tried?
Will it be perfect? No.
Is it more realistic than a shiny new system?
Absolutely!
Here's what I think.
"Legacy" is just another way of saying "we don't want to be bothered to prioritise this." And "new system" just throws the ball in some one else's court. Let them deal with it.
The bottom line is that your legacy system is your problem. Not your users'. And if you can still push bug fixes and feature updates to that system, you can push accessibility fixes as well.