Most temporary variables are single assignment, and many newer procedural languages do have variable declaration syntax for single-assignment variables. Such as

Most temporary variables are single assignment, and many newer procedural languages do have variable declaration syntax for single-assignment variables. Such as Javascript's `let`. Another wide-spread syntax feature is for-loops over collections like `for item in list`, again being extremely easy to read and not requiring any assignments. So, accidental in-place mutation is not an issue in practice. But, in-place mutation can be very useful to express logic in an easy to understand way. Coincidentally my main language does not have any of these features, and in-place mutation is like 0.1% of all my problems. (And notably, memory management is < 5% of my problems). These are all just theoretic problems, what really matters is developer experience. You can write terrible bug-ridden convoluted code in any language, ESPECIALLY with many filters and lambdas and callbacks. The issue with the functional style is that it becomes harder to see the control flow and run-time characteristics. I'm sure functional style is great to express set operations (like DB queries) but IME they tend to optimize for source code brevity and pessimize for runtime performance.