I have to agree, I'm not a big fan of the low dive number requirements put up by PADI, and other agencies.
This is what SubMariner is talking about (BASHING). Nearly all the major training agencies have similar low numbers with regards to when DM training can start, but many of you just have to type PADI.
From what I understand, 5 years is the average time for instructor burn out these days. I think a lot of factors play into that, lack of total experience being one of the factors. When you are a DM or instructor, at best you are "diving alone". You can't rely on the students to bail you out of there is a problem. You need to be comfortable enough with your own skills first.
It's not really a "burn out" it's a "credit limit max out." After 5 years of working full time as a dive instructor, most have to take a "real job" to afford car insurance and the minimum payments on their credit cards!