I have an iPad and there's plenty of stuff I literally can't do with Safari on it right now. However, it's not everyone else's fault. Yes, some websites are stupid and do UA checking and intentionally block people on some browsers, but none of those websites are the problem, as all of those sites are not going to give up a vast quantity of iPad users even after Chrome and Firefox become available. It's not like macOS because iOS is not a minority in the world of smartphones, and iPadOS is definitely not a minority in the world of tablets. It's not like Internet Explorer or Edge because it doesn't suck shit through a straw.
The websites that are actually a problem? The ones that don't care. The ones that really could not give a shit less if it runs on your stupid iPad or not. Those are the websites that you can't use on iPadOS. And while the top 500 most popular websites are never going to act this way, this mentality is more widespread than you think. They do not care, and will not care, that Apple doesn't let you run an alternative browser on your tablet. Therefore, you simply miss out.
In addition, some web apps will just not be able to have good support on Safari because Safari just generally takes longer on standards. So instead, you will just not see support for Apple mobile browsers at all. Again, you come upon it a lot. It's not out of malice for iOS users, its that Safari just isn't as good at keeping up with web standards as Firefox and Chrome are. And yes, a lot of web standards suck, but a lot of them also don't and are made to resolve real problems with the web app platform, and look, if Apple's argument that you don't need sideloading is "just make webapps", then I think it's only fair to say that it's not a very good option when they have such inferior support for the standards that people are making to make better and faster webapps.
Apple's influence on web standards has always felt petty. They're the only company that has the specific kind of influence that they do because users can't simply opt to use an alternate browser; they compete in a totally different way than almost every other browser. They should have to compete on a level playing field. I reckon that suddenly, WebMs will start playing on iOS and iPadOS.