I think this is a fascinating and deep question, that I ponder often.

I don't feel like I know all the answers, but as the creator of Replicache and Zero here is why I feel a pull to the web and not mobile:

- All the normal reasons the web is great – short feedback loop, no gatekeepers, etc. I just prefer to build for the web.

- The web is where desktop/productivity software happens. I want productivity software that is instant. The web has many, many advantages and is the undisputed home of desktop software now, but ever since we went to the web the interaction performance has tanked. The reason is because all software (including desktop) is client/server now and the latency shows up all over the place. I want to fix that, in particular.

- These systems require deep smarts on the client – they are essentially distributed databases, and they need to run that engine client-side. So there is the question of what language to implement in. You would think that C++/Rust -> WASM would be obvious but there are really significant downsides that pull people to doing more and more in the client's native language. So you often feel like you need to choose one of those native languages to start with. JS has the most reach. It's most at home on the desktop web, but it also reaches mobile via RN.

- For the same reason as prev, the complex productivity apps that are often targeted by sync engines are often themselves written using mainly web tech on mobile. Because they are complex client-side systems and they need to pick a single impl language.