The Ethereum protocol was designed with PoS in mind and has a built-in difficulty bomb[1] to prove it. In short, this difficulty bomb makes it exponentially harder to mine ETH over time. The goal of this feature was to encourage all participants of the ecosystem to transition to PoS as quickly as possible.
Given that, the implementation has not worked out totally as expected, as the difficulty bomb has been pushed back a few times over the years. However, to answer your question, the reason they did not move faster is because this transition is hard and plays in some uncharted territory.
> Note that in the future, it is likely that Ethereum will switch to a proof-of-stake model for security, reducing the issuance requirement to somewhere between zero and 0.05X per year.
It absolutely was, Peercoin (PPC), and even DPoS (BTS) predates ethereum too. What are they worth today? PoS is a scam -- also Diem is worse -- it is dystopia.
This has been the plan for a long time. But this is like Google deciding to switch to Microsoft Windows, it's not something they can just decide to do one day. There's a process. It's a massive effort and hundreds of billions of dollars are on the line.
Is it not possible there's good reasons they didn't move to proof of stake sooner? "Came to their senses" seems to imply they had no good reasons.