This seems silly to me. Especially without a rationale to back it. Just saying.
That being said, I'm OK with the fact that usernames just can't be changed.
Wrong. 50 BTC is 50 BTC, regardless of it's value in another currency. This is a Bitcoin forum, and I sometimes don't understand when people obsess with the value in another currency. If we lower it now, then we are dishonoring everyone who has donated in the past.
I think you are wrong again! We always look for value not the numbers. Worth of 50 BTC in 2010 is definitely not same as in 2015. So if admin decrease the donation amount, there will be no harm done. This will never discourage previous members as the price equivalent is same. The value is same. Otherwise you'll have to buy a pizza at 50BTC now if you had bought one in 2019 at that price.