Reality
While it is a nice idea that all Flight Instructors are only Instructors because that has always been their first aviation career choice , the reality is the complete opposite , the vast majority are Instructing in order to gain experience whilst doing ATPLs , M/E CIRs then applying to either Charter , Regional or Domestic RPT companies.
Which is not to say they are necessarily poor instructors , you will find the normal variation as per any group in any occupation .
But most do move on within two or three years and that depends entirely on airline hiring , and of those who remain instructing in the long term.... well they usually wish they had moved !
I reckon it's a very good thing if your instructor has actually worked outside the training environment for a period , be it Charter , airline RPT or any other commercial operation and can bring that valuable experience back into the flying school however it is a rarity .