I'm torn. I don't think the aim should be "become a ${framework} developer" but just become a solid programmer with specialization in front-end technologies. If I needed to bounce out of front-end and into another tech role, I wouldn't have too hard of a time because I'm familiar with more than Javascript (I started learning programming with Ruby and Rails, but got my first professional gig doing analytics/seo).
Heck, I feel like once you are a programmer, you are a programmer.
Even after a decade of programming, it still takes me a few days to learn new concepts/libraries.
My thing is, if you already know it, copy paste and edit. If you dont know it, build the code for the first time. I feel like this is the difference between a 10 year programmer and 2 year programmer.
I have the foundation for sql databases, so it would take me mere moments to write a new full stack if I already had the code.
This is true to some extent, but sometimes you really do need a specialist. Some fields, such as databases and machine learning, are deep enough that there is a huge difference between someone learning it on the job and someone who has been doing it for 10 years.
I think it also might be easier to get interviews and job offers for specialist positions where there's less demand. I'm not prepared to make a strong case for that, but I did feel like selling myself as a Ruby on Rails developer made certain portions of technical interviewing a lot more straight forward than when I was interviewing for "I'm a software engineer" roles. Just a feeling.
I feel like this is a more compelling tech tree (not mine): https://github.com/kamranahmedse/developer-roadmap