Can't you just... program that stuff yourself? By relying on such "cutting-edge" features, you get all the drawbacks of web technology with none of the reach.
No, HTML imports are the kind of technology that are the perfect architecture for this kind of web app and arguably are what make it possible. It'd take a whole blog post to explain, hopefully I can write it up soon!
There are web applications that are way more demanding than this, that have run on more platforms for a while and that don't need any of these questionable experimental features.
I could understand if you said you need WebGL or WebAudio or some other hardware/OS interface. You don't need HTML imports.
Look, I understand how one might like HTML imports. Once you've gone down the rabbit hole of Web technology, all the fancy experimental features make it look like HTML/CSS are now adequate. They're not. It's a trap.
The real answer is writing Javascript (or something that compiles to Javascript). Always has been, always will be. Don't trust the web consortium to specify things that maybe browser vendors will implement. Do it yourself now, thank yourself later.
We took the same bet with CSS grid, and it paid off. Lots of features can make a massive improvement to the development experience. We used a wide range of experimental features starting around 2014. Most of them came to fruition. HTML imports has been the only tricky one so far, which I think is a pretty great result on all the bets we took.