I think it's unrealistic for any engineer to remain highly proficient in many of those things simultaneously. I use AWS day-to-day, but haven't used GCP day-to-day in 5+ years - should I realistically be able to keep abreast of the vast number of GCP changes that have happened over that time (plus all the changes in devops that make the workflows I was using 10 years ago look outdated)? Many projects don't need Websockets or Eventsource - do I have to keep that knowledge top-of-mind across the years that I don't use them? Can you really claim that you're "very familiar" with that web framework that you used two jobs ago - assuming there haven't been a bunch of major changes since then? I guess my point here is that proficiency teaches you some knowledge, but perhaps more importantly it provides you with context, guidance and direction when picking up new stuff (or catching up on old stuff).