As an initial comment on the issue, I would say that what is unsustainable is too many subscription based MMOs. The majority of gamers aren't going to shell out a monthly subscription for too many games, so you are looking at a finite market. It is much more sustainable to have a group of AAA titles people are willing to pay for, and then a subgroup of free-to-play titles and other alternative payment systems. I definitely like the idea of the pay as you go system suggested, whereby you only pay for actual time in-game. However, it will be interesting to see how that dynamic cuts against the people who play tens of hours a week.