In what era spinning up a PoC required a week of work? Especially on the web. I've been a developer for roughly 20 years and that has never been the case, to the point that I believe people impressed by LLMs are the same who had a very low productivity. Today we have game jams as short as 3 days and talented people are able to produce very good PoC, with some almost complete!
1) It depends entirely on the concept you are trying to prove and how experienced you are in that domain.
2) Not every team will have someone with 20 years of experience in a particular domain eager to spin up a PoC.
So what you're saying is that all PoC's are guaranteed to take less than a week of work.
What are you even saying? Are you aware that there is a massive range in the scope of projects? You must work on some incredibly simple CRUD apps if this is your take.
These people work mostly in CRUD apps and they're telling you they how feel productive. Btw exploratory ideas even for hard problems come out already after a hackaon of a day or a game jam of 3 days