I disagree. I mean, it depends on the kind of app. The data requirements of some apps might do well in a simpler model that can work OK with existing syncing se

I disagree. I mean, it depends on the kind of app. The data requirements of some apps might do well in a simpler model that can work OK with existing syncing services. But the majority of apps need more complex needs and you definitely should not go and build a new database. The user experience will end up being flaky (syncing problems) and/or slow (the database is not good). Not to mention that now in addition to setting up the app, the user now needs to go sign up for one of those services if they haven't already. With integrated syncing, the user just fires up the app and syncing works and they never have to care about how it works. There's a reason a whole field of research exists into things like CRDTs. Distributed apps are not trivial, and only ones with simple data needs could get by without a more rigorous approach.