While I don't like the new Outlook much, half the time I use it is through a browser on my Linux box anyway, when I check work email from my own hardware...

I'm also don't agree with the assertions that you cannot address performance issues in a web based application. The actual email rendering in Outlook classic is still an embedded browser engine. Now it's just doing more of the application. If anything VS Code and the underlying editor system should show that you really can create pretty responsive applications on a browser surface... no, it's not the fastest, but if you're comparing to a lot of "full" IDE applications, it's a massive improvement... Visual Studio around 2010 was absolutely horrible for building web based applicatons, with common freezes and input lags.

There's plenty of room for improvement in these applications... I think the web shift is more to support cross platform better than it is to avoid optimization. At this point, they really want to be able to support Android, iOS, Windows, Mac and maybe unofficially Linux. The browser is the best bet to make that happen. I think that wasm can bridge a lot of the performance issues where service interactions go beyond the immediate state. With the rust ecosystem as good as it is for wasm target usage, I'm frankly surprised more of the logic isn't already there instead of JS. That said, I don't know how much is effectively asm.js or electronic translations to JS, or relying on server functionality. I haven't dug that deep.

That said, there's plenty of room to optimize web based applications. Even if it comes down to single-channel RDP application shells to a server-running application remotely. I think the point is to share as much as possible and support as broad an audience as they can. I think this is still happening in a context where three are still those that are trying to keep Windows on top and ignore Linux, while others in the org want to fully embrace it.