personally I always looked at group fitness as an add-on to my personal training. There is value of being exposed to a large group of people. I have never seen a group fitness instructor, though, who was able to make REAL money with group exercise even if many classes a day are taught (which also increases the risk of injury exponentially). I only teach MELT classes which is a as yet rather little known specialty.
What I observe with my group ex colleagues, though, that their value goes up when they are able to teach many different modalities, in fact, it is a pre-requisite to being hired. As Ariadne pointed out, all those extra certifications cost a lot of money, and I sometimes wonder whether some instructors really make a cost/benefit analysis. Mind you, the fun of teaching a class is a factor without monetary value but of great importance.
I assume that you already love Pilates and are drawn to it from that perspective. If the market will bear it, it is still a specialty worth looking at. I know that Pilates instructors in some places are not getting any more compensation than other instructors.