More CFI's
Every year there are more and more CFI's and fewer students. In the 1970's a CFI could earn a good living. At the time there were something like 30,000 instructors for 212,000 student pilots. Now there are over 90K instructors and somewhere around 74K students. To me this is evidence that fewer are able to make it out of being a CFI and those who do are not so willing to let their CFI expire for fear of loosing their job.
I do not think it is a viable way to build time anymore either. The fact that there are so many CFI's leads me to believe that there are a lot of unemployed instructors out there. There simply are not enough students to go around anymore. In order for there to be a healthy instructor market there should be 10 to 20 students per instructor for a CFI to be able to make it to 1000 hours of instruction given. Currently there are about 1.3 instructors per student. The numbers just do not work.
Consider putting the money that it would take to get the ratings into Avgas instead. Buy a 150 and fly the time off over of buying a CFI, IA and MEI. Imagine how hard things will be once the 1500 hour rule goes into effect. I imagine that new student numbers will fall off sharply. Flight instructors will be paying for the privilege to work.
Skyhigh