Unpopular opinion, functional programming style got a big push because the inability of languages and compilers to deal with state especially mutable state, so they bring out the purity big gun and ban all mutable states. However, state and mutable state are natural and useful for programmers to work with, thus functional programming never gains mainstream.

With the advance of lifetime analysis, mutable value semantics, and local mutability, there’s no need to ban mutable state outright. The push for functional programming will be waning in regarding to purity.