The thing is, I've never known a startup to stop development after a while and say "you know what, now we need to rewrite this using the right tech stack". That day never arrives. Let's imagine you try to write Adobe Photoshop in Ruby, or a new blogging platform using C. Both will result in terrible outcomes. Tech stack is important, but there are probably quite a few reasonable choices given any problem. In your case, Spring and React is a very safe Web app stack and I'm guessing you already knew that.