Things are moving fast at the moment, but I think it feels even faster because of how slowly things have been moving for the last decade. I was getting into web development in the mid-to-late-90s, and I think the landscape felt similar then. Plugged-in people kinda knew the web was going to be huge, but on some level we also know that things were going to change fast. Whatever we learnt would soon fall by the wayside and become compost for the next new thing we had to learn.
It certainly feels to me like things have really been much more stable for the last 10-15 years (YMMV).
So I guess what I'm saying is: yeah, this is actually kinda getting back to normal. At least that is how I see it, if I'm in an excitable optimistic mood.
I'd say pick something and do it. It may become brain-compost, but I think a good deep layer of compost is what will turn you into a senior developer. Hopefully that metaphor isn't too stretched!