I ask because I picked up where the TS and Roslyn teams left off. I actually brought red green trees into JS.

My finding is that the historical reasons against this no longer seem to apply today. With monomorphic code style JS has close enough to structs. Multi threading is now essential for perf.

I don't even think multithreading is the strongest argument for immutability, because it's not only parallelization that immutability unlocks but also safe concurrency, and/or the ability to give trusted data to an untrusted plugin without risking corruption