Do you think POS will weaken decentralization?
If you are talking about bitcoin and altcoins, there is nothing that will weaken decentralization. But if you are talking about altcoins, I do not know but there are people that prefer decentralization. If you are talking about if PoS is not decentralized, yes it is not.
Does switching from POW to POS significantly reduce gas costs? And significantly improve TPS?
No.
Why do you think ETHW failed?
Because ether and ERC20 has gained ground already and influenced by their creator like Vitalik Buterin.
If there is a new coin that tries to re-provide POW ETH, will you buy it?
Bitcoin is PoW and has not changed, you can go for it.
I found several POW-based ETH altcoins released recently, do you think these altcoins have a future?
There were somenthat were successful in the past, there are some that are created nje that will have future. But just that developers if today are lazy and only go for token instead of a coin.