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

If he had invested the 50BTC 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.