This is an excellent proposal, I don't know how technically feasible it is, but it is definitely worth considering.
I don't think so because it breaks the core value of Bitcoin Protocol. It's your private key, it's your bitcoin.
It will never happen!
Even if you increase the age, even if you decrease the amount taken by 50 or 90% no such proposal will ever be accepted.
The main problem is not what it does itself but the fact that everyone will see it as opening Pandora's box, if this happens then what's next? And people's imagination will run wild, what if next it's a mandatory annual tax of 0.1% on all coins?
Furthermore, you have Satoshi's own opinion, lost coins are a donation, he had no plan to recycle them.
I agree with you.
Bitcoin is our own banks, with private keys, or mnemonic seeds. If there is a term like this in the Protocol, and a code to refund our bitcoins after 50 inactive years, people will lose their faith in Bitcoin.
The threshold is hard to define too. 50 years, people have longer lifespan that it and even threshold is 100 years, there are people can live longer than that. More important, Bitcoin can be used as inherited asset for our children and family members, from generation to generation.
This idea breaks all of good core value of Bitcoin Protocol.
It is a Pandora box we never want to open similarly to increase Bitcoin total supply.