I did in my last job change - and 80% of the office here didn't know the stack/language when they first came in.

> Secondly, if someone has to learn the language when they join that puts a lag on how quickly they can be useful.

You could learn enough dart to be useful in a day if you are a C#/Java dev with JS experience - enough to go through the codebase - and thats something that takes time no matter what the language - dart isn't that exotic.

Flutter has a higher learning curve but that should also be on the order of week or two depending on how much you know about frontend. React native is the same deal.

> And lastly, companies are (usually) very reluctant to let someone spend the first few months getting up to speed if a technology choice means they can recruit people who'll immediately be effective.

Meh - when they chose stuff like Dart they know hiring pool is limited and usually word the job posting like that - ie. they expect you to be able to pick it up on the go.

I mean your whole argument is nonsense because things like this happen all the time - my current gig I started as a .NET dev with some python/django experience - I'm working on ruby on Rails stack - never touched that before in my life. Took me like 1 day of reading up to be able to go through the codebase - a few weeks and code reviews and I'm pretty much in tune with how they write RoR.

That could be because experience with that language is often listed as a requirement.

>someone spend the first few months getting up to speed

In my opinion, "few months" is a huge overestimation, if we're talking about an otherwise experienced mobile developer who understands the underlying platforms (Android, iOS) well. A few weeks should be enough to get up to like 90% of their usual productivity (ballpark guess).