You don’t need to go full FP to experience the benefits. You can just apply FP principles on a small scale to sections of your code, where appropriate. As for

You don’t need to go full FP to experience the benefits. You can just apply FP principles on a small scale to sections of your code, where appropriate. As for “many big applications”, it depends on what your threshold for “many” is. There are a number of firms that use e.g. Haskell, F#, or ML privately, e.g. Jane Street, Galois. For whatever reason, functional programming seems to be more popular in finance.