I'm trying to get a high-level understanding of what percentage of internet users allow their browser to store the username/password so I can understand the potential impact "breaking" this will have on my customers. Been Googling all day and not finding much...any help? Thanks!
Here's an article from Pew Research that points out which method is used by users to save their credentials from a survey in 2016.
[W]hile 18% say that they save them using the built-in password saving feature available in most modern browsers (with 2% saying they rely on this technique the most).
For future, such statistics related answers, you'd most likely find a Pew Research article/study. Hope it helps.
I believe most of the users save the passwords. Way more than 50%. I have no formal statistics on that, but by watching many persons navigate I now this to be common.
You should only break compatibility if you have a very very good reason to do so.
To keep the saved passwords working you only have to keep the login form URI and the form structure. You can rewrite all your site, even the login subsystem without messing with that. Even if you are changing the framework, little changes in the new framework will allow to use the same login form URI and the same form input name.