As you say, good TKIs are getting harder and harder to find. Flight Engineers and Navigators are, quite literally, a dying breed, and I see a significant rise in the use of newbie pilots, who can't land a flying job, being used as stop gap TKIs..... until, of course, they get that right hand seat job!
The level of real-life experience of TKIs, is diminishing rapidly. I suspect that within in a few more years, the guys with real-life experience of Navigation, AGK, M&B (mainly ex-military Navs, FEs, loadmasters, etc) will have retired and be very thin on the ground. What then? Delivery of theoretical knowledge only, unsupported by real experience. Or putting it in the vernacular, all mouth and no trousers.
I am a former military navigator (by the way, seeking for a TKI position) and only 36yo. They still are numerous younger than me, I guess the "few more years" you spoke about will persist for a quarter of a century.
The experienced TKIs gap has probably one or even more other causes.