Exactly. I managed to ride a nice maybe 8 year wave as an "OpenGL expert" but the industry moved on and I didn't go more general with 3D graphics to climb out of my niche. I haven't been in that industry since the OpenGL waterline crested and went back out to sea. Learned that lesson.

Be a generalist. Yes, deep specialists exist and yes, some of them have successful careers based on their deep specialty, but betting on specialization is like a high school kid planning to be a MLB baseball player.

Ruby is so god awful yet so many successful companies use it as the bedrock language there's always going to be jobs maintaining the piles of crap it leads to.

Yep. Craftspeople take their craft seriously, sweat the details, learn the patterns, go deep.

And that’s complimentary with marketing oneself as Ruby/Rails dev or whatever else the market needs at the time.