Just my $.02…
I think that (as rug3y points out), there will be some situations where being a generalist will be a plus.
Look at it this way. As a professional musician, I always have people coming up to me and saying, “Hey, my uncle Mike is the best musician ever! He can play every instrument!” And I always am left feeling, “Uhhh, so what? When has anyone ever been hired for playing every instrument?” You get hired because you can play one instrument well.
Or to change the metaphor, as a musician I’ve sometimes been hired because I can play just about any style. In that sense, I am a musical generalist. There are a few jobs (like on a cruise ship) where that comes in handy. But, the vast majority of jobs I get are because I play jazz guitar very well. They don’t care that I can fake my way through a country solo or can play reggae or I can play mandolin. The vast majority of work is because I’m a specialist.
To go back to my original metaphor, there probably are situations where playing multiple instruments is a good thing. Band teachers have to. It’s a good skill for conductors, composers, and arrangers. Playing music for a circus…
If you want to be a generalist and have a few jobs open or work for yourself, then go for it. I mean that seriously - you can build a good career that way. If you want to open up some new possibilities, then choose some favorite techs and dive deeper. You can be a strong candidate who has a solid base in a wide range of techs but has a focus on one in particular.
In reality, that’s probably what most specialists are. If someone is a React specialist, that does not mean that he’s never done jQuery or never tried Angular or whatever. Probably most specialists have exposure to a wide range of techs but simply focus on a couple.