As there are less and less tokens being minble nowadays compared to 3-4 years ago, what happens is they all decide to go POS
This has been addressed by Laurent. MMU's mine the most profitable coins. If ETH or any other move to PoS, there'd still be othe coins to mine. Also, for the long term, they might offer hosting services using the MMU.
Definitely an issue, I think its one to worry about when the time is right, maybe in a few years.