This is an interesting perspective that I haven't heard before. Two things jump out at me here.The Transfandom as a whole is getting older. And, there are quite a few among us who are stereotypical "fat guys collecting toys". I know that a few have died in the last few years of expected complications. (That is not counting guys who get random maladies like cancer and such, or die in accidents.) I expect the next 10 years to be high-attrition for the fandom. (And, I know a couple of guys who are likely to be in that demographic unfortunately.) Joe fans, being anywhere from 5-10 years older on average are not likely to fair much better.
What exactly do you mean when you say "The Transfandom"? Are you referring mainly to the older fans, say late teens and up who have moved past the age where they play with toys, and who strictly view this as a collecting/social hobby? People like me, for example, who had Transformers as a kid and for whatever reason got back into them in my late 20s? It's correct to say that segment of fandom is aging, but do you really see fandom contracting as the older fans either leave the hobby behind or, sadly, pass away for one reason or another? Maybe that's not what you meant at all, and maybe you were just referring to that older segment. But that's how I read it.
I don't think you can discount the kids who are being drawn into Transformers today, of whom a portion would presumably retain interest and form the future fandom. I think at some point the look of Transfandom will change greatly, but then wouldn't that be the case with anything? The older guys will move out and move on, but there will still be enthusiasts for the hobby.
Tangentially, at what point does it become awkward for an older fan to be involved in this hobby? We're adults who collect toys, which is fine up to a point. But is it just strange for someone to hit 30 or 35 and be discussing the finer points of a paint deco with someone who's 15 or 12, for example? In future, could we expect to see the fandom segregate itself into older and younger groups to deal with this issue?