So it should just be backported to npm to show that the authors are serious about backwards compatibility.
I was OK to merge pnpm into npm in the past. They have never suggested me this opportunity. Instead, they decided to re-implement pnpm's algorithm into npm and call it "isolated mode".
Most of my problems were created by the material UI libraries (I wanted to use them with SvelteKit), but I just got rid of it as those libraries were making development harder instead of helping.
I still wish there would be a nice UI library for Svelte, but I guess that's the disadvantage of not going with the mainstream frontend toolkit.
I'm glad I'm not the only one who has had that experience.