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Sure, it's definitely something worth taking into consideration, I just wouldn't value it too highly and in many cases I think there are much better indicators of ability.

> I imagine the developer pool is much smaller but the candidates are higher quality

People always say this about their favorite niche language and it always feels a little too self serving with little evidence to back it up. Learning languages is relatively simple and enjoyable, especially when the overwhelming majority of those people never end up working on difficult problems in said language. I think it often gives a false sense of superiority as well.

> but the longer your repl session is open and the more you change and compile, the more likely you are going to end up with a mixed state in memory that does not reflect how your program would behave if run from scratch.

I've seen this happen multiple times including during live Clojure talks at conferences. I also never found REPL driven development all that beneficial outside of quickly toying with trivial pure functions and that's possible in basically every popular language these days.

Extremely shallow criticism based off of little more than personal anecdotes.

In Rust you can solve this via derive or a macro. Typescript's conditional types and the associated utility types are a nice solution to this problem as well. You can do a ton with Partial, Required, Pick, Omit, Exclude etc.

How exactly are you going to implement a type safe option type without generics? You either have to use interface{}, which is obviously not type safe, or write/generate an option type for every type contained within it. Sum types are a lot less useful if you don't have generics.

> Is that not what generics would fix?

No. That would be fixed by having something akin to typeclasses, traits or interfaces.

Elm's List, Maybe and Array are all defined using a generic type parameter.

> Well, I just come from a Elm gig and I must say I was not too bothered with the lack of generics there.

Elm has generics.