> nobody knows whether something will become the next Google Docs

How many times has it actually happened that some scrappy startup has 1) became the next big thing and 2) not being at the edge of over-engineering actually killed it or significantly impacted its revenue? This just feels like wishful thinking.

Also keep in mind that even if you were on track to become the next Google Docs, this means your current product is usually good enough as-is and gives you time (and $$$) to improve it.

I'm not sure that using React or another JS framework counts as 'being at the edge of over-engineering'.

I agree with the rest of your point - the value of the product to end-users has little to no correlation with the underlying technology choices, which is a pretty controversial statement, but one that I think is true. A customer doesn't care if you built it in React, in one Perl file, or if you're sacrificing goats to retain the minimum requisite levels of dark magic to keep the system running. If it solves their problem they'll keep giving you money for goats.

It depends on what the objective is. I've seen plenty of project where React was used just to have it as a buzzword, but otherwise provided no functionality and actually slowed development down and ended up being less reliable (we had to - poorly - reimplement behaviors like validation, pagination, etc that our backend framework already had for free).

Another view of this problem, evolving a SPA of CRUD app into Google Docs may also be a complete rewrite.

IMO when the time comes and your product is used well, you may be ready financially and technically to do a complete rewrite. Otherwise maybe the current functioning application is better if the rewrite isn't justified.