I tend to agree - playing one type of poker, and focussing on being the best you can be at it, is probably the best way to go.
The idea that you
need to be good at every form of the game to consider yourself a true poker player is an outdated idea IMO. It goes back to the days of the road gambler, long before the internet, where you played whatever game you had access to in town that night. So you needed to be at least passable at everyting, from stud to omaha to deuce-seven, because if you weren't you weren't going to get a game that night.
Ever since
online poker came along though, that hasn't been an issue any more. You've got access to pretty much unlimited action in whatever game format you like. So if you're happy playing just the one game, by all means do that.