There's a huge difference between chasing fashion and seeking out what is new that is actually better.
I agree that those people are usually worse. The ones that pick up on new stuff that isn't necessarily fashionable, assess it intelligently and balance the trade offs of the good vs the bad - those are the better programmers.
IME scala programmers are better - mostly because they tend to be programmers who got frustrated with java's shortcomings and realized scala solved many of those shortcomings, not because it's fashionable to learn scala.
IMO people who chased mongo, on the other hand, demonstrated a responsiveness to large marketing budgets rather than critical thinking skills.
You don't have to try out every new technology either. There are some that you just have to read a bit about to know enough to dismiss them.
As someone with extensive experience in both Scala and Mongo, I agree. Mongo is nice for experiments that might go nowhere, but when you give it a real production workload, it falls apart fast.
It's pretty easy to find people who use "boring tech" and still fall prey to the hazards of overengineering, slow delivery, etc.
If they are doing it to experiment with a new library/framework then finishing may not be the point; it could be to see if the tool is worth the hype or if it adds unnecessary complexity.
This exploratory tinkering has been good for me personally because it has developed my sense for parsing out the bullshit used to market new tools.
If I had to choose between the guy who's done 10 years C# and the guy whose done 2 years C# and 2 years JavaScript, I'd pick the latter all the time.
As a friend of mine put it, "for most people, a 10-years experience is 10 times a 1-year experience".
In my professional experience, people that are always learning new languages or are constantly using new libraries and frameworks are never the ones who accomplish the most. They simply are the ones who are always seeking novelty; they never become great at a language, because they are always wanting to try something new. They start lots of projects, but don't finish them.
I don't think those are the best qualities to look for in employees.