I think cloud-first design is natural because webapps have nowhere good to store state. On Safari, which is the only browser that matters for many web developers, everything can be deleted at any time. So if you don't want to have a horrible user experience, you have to force your users to make an account and sync their stuff to the cloud. Then, the most natural thing to do is to just have the user's frontend update when the backend updates (think old-school fully-SSR'd apps). You can do much better than that with optimistic updates but it adds a lot of complexity. The gold standard is to go fully local-first, but to really do that right requires CRDTs in most cases, which are their own rabbit hole. (That's the approach I take in my apps because i'm a perfectionist, but I get why most people wouldn't think it's worth it)

With the files API, apps could actually replicate the microsoft word experience of drafting a file and saving it to your desktop and praying that your hard drive doesn't fail, but despite offering great benefits in terms of self-custody of data it was never a great user experience for most people.