You certainly don't have to play cash games. There are lots of poker players who have made a career from MTT's, or SNG's, even specifically HU SNG's.
You just need to be aware that the strategies for each of these games is unique to itself, and therefore I would suggest you invest the time to learn whichever game is the most enjoyable for you.
Most people will never make poker a career, but many can enjoy the game, and even make a little money at it.
With that said, I feel that micro stakes cash tables are probably one of the lower variance forms of the game. MTT players can go long periods without cashing from what I understand, and for me anyway, SNG's are just not worth the time/heartache. These things can easily drive you insane, and if you get any good at playing cash, then the $/hour you make is just sick compared to being a nasty SNG player.
BeastieMik talked about this from experience, he was a very successful hi stakes SNG player, now he plays strictly cash and says he wishes he hadn't spent that earlier time playing SNG's.
If you're decimating people at any level, I'd suggest continuing to do that, and soon enough your bankroll will be much larger.