The problem with proportional pricing is, who decides the value? If your website becomes successful, and the word "facebook" becomes more valuable, what's to stop ICANN simply increasing the charge? Then it just becomes extortion from ICANN.

The new-generation TLD owners already do this with "Premium" names. Google does it with .app. A price is arbitrarily decided for your first year. Who knows what price it might be in a few years' time.