applicants that had extensive SEP experience as they 'would find it difficult to adapt to jets'
This is nonsense mate, absolute nonsense. Being a flight instructor will give you a lot of key skills that will be to your advantage being an airline pilot.
Flying jets is simply different to flying light aircraft, and once you have secured your first job the airline will give you all the correct tools to be able to be a competent first officer.
There is no doubt that the whole system of sliding into the right hand seat of an airliner has changed, and self improvers are a dying breed, however, you will do yourself no harm from instructing.
All the best with it mate.