IE6 outlived it's expected lifetime by leaps and bounds due to lots of customers stuck on machines locked into running old version of windows due to financial or corporate policy reasons.
(Fun spitballing doomsday scenarios: LineageOS gets more traction and needs to harder fork; or, Huawei is forced into a hard fork and takes the entire Chinese market with it, which could snowball other markets across; or, some Android worm causes friction with the major carriers and they get back into the version/upgrade micromanagement game even worse than before.)
Today isn't a dreamland, but it is in no way as bad as the IE6 days.
But in the early 2000s, you only ever targeted IE6 and maybe 5. Opera was that weird European thing, Netscape disappeared by 2003 and Firefox was that silly open source stuff used by people without friends or any sort of social skills. That was the IE days.
Just like nowadays more and more pages only get coded for chrome and might or might not work on Firefox.