That is not entirely true. After learning the n-th stack and knowing the knowledge will likely sputter out in a couple of years cycles, one is likely to learn enough to get done what needs to be done. As someone points out, few out there are true experts if they're constantly learning a new stack. And also people who want to learn all the time do get bored of learning the same thing, a lot of them branch out in different directions that are a lot less shapeshifting.
Ideally I preferred learning the technology for a few years then steeping back and reaping the benefits and do stuff with it. But, there is a fear that not keeping up leads to obsolescence on the job market so we're in this constant cycle of learning new things with diminishing returns
First, I accept that there are cases where extreme specialization is important. I just think these cases are few and far between.
The main skill for senior/staff swe is not in knowing or not knowing some technology, but rather
1. Picking things up quickly
2. Understanding/recognizing generalizable patterns and best practices in any tech.
Almost no job requires some extreme expert level knowledge of language/framework minutiae (obviously you do need some experience in particular tech). Almost every non-greenfield job has a ton of custom tech that has to be learned. Almost every green-field job needs someone with immense breadth.
Having been exposed to very many things people get to naturally see recurring patterns/best practices. When they see a new tech they are able to understand why choices were made. When new tech needs to be made they can draw from broad experience in what others did best. I really think this is most of the eng value of staff swe.
Ideally I preferred learning the technology for a few years then steeping back and reaping the benefits and do stuff with it. But, there is a fear that not keeping up leads to obsolescence on the job market so we're in this constant cycle of learning new things with diminishing returns