I've worked with a customer who also had a post-production environment. They used it for the sole purpose of being able to replicate problems and do root-cause

I've worked with a customer who also had a post-production environment. They used it for the sole purpose of being able to replicate problems and do root-cause analysis in case things went horribly wrong. Then they took a snapshot of prod, synced it to post-prod, hotfixed prod as fast as possible, and then did their detailed analysis in post-prod. This wasn't cheap; they payed Oracle somewhere between 50k€ and 200k€ a year just for the database for this environment, but they considered it worth it. (They were also in a pretty tightly regulated vertical). My main takeaway is that I don't think there is a one-size-fits-all answer to the question of how many and what environments you need. IME having at least one "buffer" between dev and prod is a good thing, but I'm not sure to which extend my experience generalizes.