Honestly when they come with decisions like that, I'd like them to spend some time on a formal writeup - have them prove they understand the problem, the existing solution, the issues with it, and why React or technology X would solve it. Have them explain why they / their employer should spend thousands on changing it.

I mean it'll only take them an hour or two to write it up, better to spend that than the 20 hours it would take you (for example) to spin up a new stack.