We can very easily keep backwards compatibility forever at no ongoing cost by layering compatibility mechanisms on top of newer standards. No reason to deprecate <blink> or <marquee> if you can just express them in terms of CSS animations in the default stylesheet. But most of the time it makes sense to pay a bit of maintenance cost because it is worth it to actually improve the ways we access old content rather than just not regressing.

In fact, often backwards compatibility is what keeps things from getting worse because the current environment would have never allowed something as good if it wasn't already entrenched. A good example of this is EMail - big tech has no interest in interopability and aside of upcoming laws forcing them there is no way that a federated communication system like EMail would have any chance today.

You can't safely use old browsers on the internet. Heck, you can't really safely connect old anything to the internet.

There are lots of stuff out there where the browser cannot be updated.

What do you mean, Windows is getting better and better :P