I think the current crop of sync engines greatly benefit from being web-first because they are still young and getting lots of updates. And mobile updates are a huge pain compared to webapp updates.

The PWA capabilities of webapps are pretty OK at this point. You can even drive notifications from the iOS pinned PWA apps, so personally, I get all I need from web apps pretending to be mobile apps.

Yes. PWAs only need now Dev adoption. It is up to us to fight the App store monopolies.