You hit the nail on the head (well, half) - design is an incredibly tricky problem.
Mostly because it at its core is the process of adding constraints, and once you've added those its far harder to un-add them. That's why CSS was so janky for so long - there were design paradigms (tables, box model, flex box, etc) and once they were there, you hoped they worked and you hoped that your specific design didn't break one of its core assumptions, leading to large amounts of work-around code that was either slow or exhibited constant random bugs in different versions of browsers...
The other half of the nail was backwards compatibility. Unfortunately you can't have web components and your stuff working on 20 year old devices. So any new design/paradigm etc. served to splinter the ecosystem (your stuff only works on this browser, that device, our operating system).
Graceful degradation sort-of would have provided a work around, but it would have done so by shifting the workload onto the provider (please make sure your calendar works, in these browser, and also has fallback implementations as a clock that doesn't break your back-end code because they entered hours as farsi, test on on these 40 browsers and also target moving backends). So instead we invented polyfills as a way of "automatically" back-filling for a lot of stuff.
Unfortunately, you can't do much about the Web Component calendar. The design paradigm that once existed doesn't support it without a lot of busy work, even if newer browsers could natively implement one and never load a line of javascript or any of your shitty thousand line long html layout code...
I don't think its a completely lost cause, HTMX + Web Components seems like our best path forward so far. Right now, a lot of people who've written millions of lines in non-html frameworks like React and the like are very reluctant to give up their codebase(s), however terrible they might be. It is unfortunately, the result of someone somewhere making a design decision that added constraints, now causing headaches and difficulties un-adding those constraints...