Developers trying to justify their employment and pad their resumes.

Resume-driven development making people justify their jobs.

Come on. Are you an employed developer? If so, even if you have that much power over the flagship product of the company, that you can use it to pad your résumé, do you think the same happens at Microsoft? By all means, the bugs may speak of skill issues (or may not: there's also crunching, mandated AI... Who knows), but the Cloud-first, push-ads, force-account etc. enshittification is the implementation of a vision I doubt was collectively composed by devs, let alone a single dev.

The ads pushing would be just as possible with a fully native start menu.

Using React for it was probably done since it's just objectively easier and faster to tweak a React app than native components (see various folks complaining about WinUI).

I was addressing the concept of developers taking it upon themselves to rewrite the menu (in whatever; React is beside the point) in order not to seem redundant and to pad their résumés.

My best guess is ads.

my guess is many junior developers without guidance and perhaps a little vibe coding. could be that these new developers are not comfortable anymore with C++ and MFC so they use what they learned in their courses directed at web standards

Sounds like bullshit, I'd rather bet that Microsoft wanted to make some Cloud OS, web friendly thing and expected it to be as successful as Visual Studio Code

Also, I don't think that integrating react app into Windows is trivial