> it would be best if developers all over the world boycotted proprietary platforms entirely.

Note that Linux and Android, from your list, are open source.

> For true cross-platform, you need to develop for 5 platforms

I would argue that anyway, you should develop for multiple "kinds of devices". Writing the same code for a desktop computer and a smartphone is fundamentally a mistake: I don't want to slide big buttons with my mouse.

So first, you need to design the UI multiple times: for desktop, for smartphones, and for tablets. If you want a smart watch, you need another design for it. Web apps don't solve that better.

Second, if you can write a webapp that works everywhere, then you can write a native app that works everywhere. I don't see how that would be different. The problem is generally that if you run the exact same code everywhere, then it's not a good experience.