Well, say a company is looking for an Angular expert -- are they going to hire someone who's spent the last year doing Angular, along with 50 other things, or someone who's spent the last year hyper-focused on Angular? I think there are jobs for generalists for sure, but there are also jobs for super-pressing needs (read as $$$) for specific strengths. If you spread yourself too thin, you're not going to get the job for that one skill-set they need. I could be totally wrong, but that's how I approach it. Also note that it's assumed devs have some knowledge of other technologies, so I'm not saying don't learn plenty of stuff.