MONEY!!
That's what keeps a good instructor.
Unfortunately, in this industry, instructors are still thought of as being at the bottom of the food chain when it comes to respect/money.
I worked as an instructor for several years and loved it (still do). The problem is; after spending upwards of $40000 USD (more in the UK), everyone eventually wants some return on their investment. I think we all love flying and wouldn't want to do anything else, but we have to have a life/make a living as well.
During my time as an instructor, I saw quite a few guys come and go. Most, get themselves a thousand hours or so, and go fly a bigger, turbine engined machine, for more cash, less working hours and more respect. If you want to keep an instructor, give them all three.
An instructor that has been with a company for a long time, will have lots of contacts, respect from any flight examiners, and will know how to attract and deal with new students/customers. They will have extensive knowledge of the local area for any photo jobs you get in, and your insurance rates should be a lot less with high time, proven instructors.
I doubt things will change, but all those reasons and many more are why instructors leave as soon as possible.
Without instructors there would be no pilots.
I hear the Schweizer 300 CB, has recently completed 100 thousand hours without a single fatality. That' pretty impressive considering it's the same bunch of people that fly the R22.
Happy New Year all.
Fly safe.