Just saying you can't take a loan and say I will only repay the principal because 0.1BTC = 0.1BTC 
Exactly how are donations comparable to loans? They're not.Because both of them involve money.
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The time value of money (TVM) is the idea that money available at the present time is worth more than the same amount in the future due to its potential earning capacity. This core principle of finance holds that, provided money can earn interest, any amount of money is worth more the sooner it is received.
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Lauda's point is that fiat value is irrelevant (I agree), mine is that time factor(/opportunity cost) should be considered.
Changing the required amount at this time would be unfair to the people that have previously donated.Exactly.
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if I already knew bitcoin then and part of this forum and bitcoin was only $10 I would have donate 50 btc because it's worth to be a vip of this forum but I guess it's almost impossible to donate $50 now..
Why would it be impossible to donated $50? You can donate any amount that you want, just don't expect things in return.He probably meant it is impossible for him to donate 50BTC now.