More people think they need the latest and hottest in manual memory management than actually do.
More people think they need a hundred thousand threads than actually do.
If you do have one of those cases, by all means prepare for it and deal with it. But be sure you have one first. The program that exceeds so much as a 100 threads is not only exceptional, but very exceptional. The exceptions are cognitively available and leap to mind, but are nevertheless the exceptions. And, again, if you have one, deal with it, but be sure you have one.
If you're sitting there in TypeScript land writing "async" code you've already surrendered on Ultimate Efficiency anyhow. Deciding what is more efficient between a threaded program in a runtime that doesn't box everything and JIT-optimized JS code is difficult but it isn't that hard for the threaded program that isn't boxing to win out on all runtime measurements, including consumed RAM.
"You write "index = last_index++;" and it Just Works (tm), no need to worry about locks or atomics or other thread access."
That goes back to my comment about using better threading techniques. I write a lot of "index = last_index + 1" (mutably incrementing like that in a single expression is just bad style anywhere you see it) in my threaded code all the time without thinking much about it, because I use the model where by default a value belongs to the one actor process that has access to it at all. The problem isn't that mutation is dangerous in threaded code, the problem was people writing threaded code based on a ton of threads running around shared data structures with locking. Not only is that not the only way to write threaded code, it is literally the worst. There are many other options, all of them better in some way, many of them much better.
Contrasting the difficulty of writing threaded code to something else based on the assumption that "lots of shared state locked by semaphores" is the only way to write code is like a Haskell advocate talking about the amazing benefits of functional programming while writing as if literally every imperative program is just one big pile of unmitigated, pure spaghetti code where everything is linked together with gotos and every variable in the program is a global variable. That's not the relevant comparison any more. It hasn't been for a long time. If anyone's program is scrambled because they did write a big pile of gotos and global variables or they did write a big pile of shared state with semaphores everywhere, that's on them. A vast array of better techniques of all shapes and sizes was available to them.