oldest price I could find on bitcoincharts [3] is from 2011-09-13 ~5 USD per BTC. The donations come from a time when BTC wasnt worth as much. It was discussed often if the amount to donate for VIP should be lowered, but theymos and others in charge think it would not be fair to those that donated 50 BTC in the past. I also think its a good idea that VIP essentials says: I have been here a long time and I believed in it. If the amount to donated would be lowered to e.g. 0.4 BTC (~ equivalent in USD to 50 BTC 2011) we'd see way to many VIPs.
With the price of a single bitcoin increasing every year, I think there should be a third category for people who donate a lesser amount, e.g. 1 BTC. Or perhaps different tiers such as bronze donator: 0.01 BTC, silver: 0.1 BTC, gold: 1 BTC, and platinum: 10 BTC. Existing donators would be given platinum status and VIPs would remain as VIPs.