Af15 is relatively safe. Tmnt is anything but safe. In fact I would call it risky at near 5 fig levels (but then I would call any post 1975 book "risky" at those levels)
Nothing is "safe" when it comes to comics. Such a small segment want them, and I wouldn't think that segment is growing.
However, your statement reflects a belief that old is a huge factor in value. And yes, it is a factor. But it's not nearly as important as supply and demand. We know the supply of TMNT #1 is far, far less than any Spidey book, AF #15 included. The demand may not be the same NOW, however, TMNT have been around for 30 years now, constantly in the public eye, in one fashion or another. All those 8-to-whatever year olds that grew up with them are now at prime income age, and just like you(assuming), me and other older guys, nostalgia will drive them to what they loved when they were young. Heck, my 7 yr old and all his classmates love them.
I think TMNT #1 IS the AF #15 for the next generation.
IMHO, of course.
The flaw, as I see it, is that while the turtles have been around for 30 years, spider-man has been around for those same 30 years (and 20 more) and is infinitely more popular and in demand and with far greater exposure and support. To state tmnt #1 is going to be the equivalent to amazing fantasy 15 is, in my opinion, a flawed reach.
Today's income earning, former 7 and 8 year old products of the 80s overwhelming will continue to love and subsequently choose spider-man. Tmnt just doesn't translate as well, nor does it hold up when one compares the two.