No one has yet created a Proof of Stake system that offers the same guarantees that Proof of Work offers. In fact, there are arguments that it's not possible [1]

You can implement any feature you want in Bitcoin, but for a feature like this (hard fork), you need to convince all users to upgrade. I think it's really unlikely you could sell a switch that drastic in Bitcoin land, especially considering the current unknowns.

[1] https://download.wpsoftware.net/bitcoin/pos.pdf

They can't even increase the block size, how would they introduce this?

The max block size for bitcoin went from 1mb to 4mb in August of 2017. The ONLY reason it took so long is that the courtesy signaling that core put in for miners was used as a veto as part of this anti-bitcoin effort to try and increase centralization. Please at least familiarize yourself with the oppositions arguments before stating conclusions like this.

8 years to change the value of a constant!

Remember that Bitcoin is about distributed consensus and for the change to apply a vast majority of users need to approve. Take any other software and see how many people still use old versions (e.g. Microsoft Office). In Bitcoin that wouldn't be acceptable for the change to work.