Why is the VIP still available for 50
BTC? Why is inflation ignored?

If he had invested the 50
BTC elsewhere, he would have had more. It should be accounted for.
Theymos will not change the pricing out of respect for the people who have donated previously, because 50 BTC = 50 BTC regardless of the fiat equivalent. In other words, if those people had not donated back then, they could have 50 BTC today.
Just saying you can't take a loan and say I will only repay the principal because 0.1
Lauda's point is that fiat value is irrelevant (I agree), mine is that time factor(/opportunity cost) should be considered.