It's a common stack, so no more or less replaceable than any other stack such as any other permutation of (Mongo/MySQL/PostGresql/SQL Server) + (Express/Rails/D

It's a common stack, so no more or less replaceable than any other stack such as any other permutation of (Mongo/MySQL/PostGresql/SQL Server) + (Express/Rails/Django/ASP.NET) + (React/Angular/Vanilla) + (Node/Ruby/Python) etc. React is as modular as the developers choose it to be. You can write convoluted code in React too. React does force you into a certain way and makes it easier to standardise, but these days with hooks and Redux and much more there can be a lot of variation in architectural styles. Picking what tech to use is hard. Sometimes it doesn't matter at all, just pick what you are used to. Case in point most peoples SaaS side projects. Sometimes it makes all the difference. If using clojure + scala + akka + kafka + prolog means you can crush the problem, then choose those, don't pick worse languages for solving a given problem so you can handle people leaving. I suspect developer replacability should never be that much of a concern in the choice of tech, where you are even considering something esoteric for genuine reasons.