Make browsing the internet possible only on Chrome, Safari or Edge (with no modifications or extensions). No competition allowed in browsers.
Make browsing the internet possible only on macOS, Windows, Android or iOS (no custom Android distributions, definitely no LineageOS or GrapheneOS or whatever). No competition allowed in Operating Systems, especially no open source operating systems.
Make crawling the internet possible only to Google. No private crawling and no competing search engines.
Let me know if I've missed anything...
Make browsing the internet possible only on SoCs allowed by Apple, Microsoft, Google. No competition allowed in SoC. [0]
Make browsing the internet possible only on form factors approved by Apple, Microsoft, Google. So no calculator with a web browser [1]. No competition allowed in form factor.
Make browsing the internet possible only on UX approved by Apple, Microsoft, Google. So backtracking 10 years ago, when Android made documents-oriented web browser (= each tab appears just like a standalone app in recent apps), that would have been abuse of that position. No competition allowed in UX. [2]
PS: I come from Android OS world, all those examples already apply to Google/Android.
[0] Well this one will depend on whether their Web Environment Integrity implementation will enforce full secure boot approved by them. Considering how it went for Android, I'd say it will, but can't say for sure.
[1] Yes you can find calculators running Android (but can't run Google/Android so no Chrome). Amongst a lot of other weird Android devices. You can find walking robots, toothbrushes, urinals running Android.
[2] You'll probably find a better example. Arguably it's the same as "competition allowed in browsers", but that was an OS-wide change, but saying it's "OS" IMO largely reduces it.
So? They can force you to pick between running old software or running new software. This is hardly new if you look at the broader "compatibility" scene. Old hardware and software are being dropped all the time. (Remember when MacOS dropped 32-bit support and wiped out a huge chunk of older games?)
If you want to stay in the old chain, you're free to do so, just like how you can still pick up a word processor made a couple decades ago and make documents on it. It only affects you if you want to use the Internet as that keeps evolving. (If you load up some '00s or '10s era browsers you'll see that many of them do not work at all for the popular Internet sites, which have all adopted things like newer TLS implementations and HTTP/3 or whatever the latest one is...