There's a big difference between changing compiler tools, and showing stuff to end users who likely have no idea what the hell is going on, and we'll just have more of these[1] situations. (Even if it's a fake comment, it's close enough to real responses from people)
Most of the sites in the world are run by people who have little to no understanding of the internet beyond the most basic "series of tubes" aspect.
If you're going to break something because you don't want to support backwards-compatibility any longer, break it in the least obnoxious way possible.
There will always be non-technical users. The trick is to focus their frustration on the parties they should be frustrated with. Take for instance those screens browsers present when you try to visit a website with dodgy security. These are universally awful user experiences and users likely have no idea what the hell is going on, but browsers do it anyway for the users' benefit and for the long-term health of the web. When users encounter these warnings, they don't think "Oh jeez, Firefox sucks so much" they think "I better get the hell out of here and not visit this web site anymore".
I'd argue that more strict adherence to standards and rejection of old, deprecated code is better for users because it's better for the long-term health of the web as a platform. The effort spent maintaining a mess of unnecessary backwards-compatibility (particularly with things meant to be temporary from the start) is effort that would be better spent providing actual value for users in the form of quality improvements, better performance, and features.
Most of the sites in the world are run by people who have little to no understanding of the internet beyond the most basic "series of tubes" aspect.
If you're going to break something because you don't want to support backwards-compatibility any longer, break it in the least obnoxious way possible.
[1] https://i.imgur.com/SL88Z6g.jpg