IMO, what may or may not happen will differ from person to person, and one of the best indicators of what can occur lies with one’s older female relatives. Looking at my late older female relatives, they all seemed to have fine, low density hair that didn’t show any signs of aging other than turning gray. Even amongst my sisters, who are 52, 57 and 61, there isn’t any receding hairlines, thinning or increased shedding. As for myself, even though I wasn’t particularly abrasive with my hair (notwithstanding the foam rollers in the 1970s, the Sun-In in the 1980s and the daily bouts with a curling iron during the 1990s), it didn’t grow as fast in my early years as it does now in my 40s, i.e., I cut my hair to SL in March and it’s past APL now. It also would shed more; I used to have big, palm-sized balls of hair at the end of each wash day, which starkly contrasts the usual quarter to fifty cent piece-sized mounds of today. IOW, my hair is in better shape now than it was when I was younger, and looking at my sisters and remembering my mother and aunts, I’m pretty confident that it will remain that way for the remainder of my lifespan.
Still, the best advice I can offer anyone with concerns for the future is to continue focusing on their hair’s health while remaining flexible to change, e.g., if someone’s hairline starts to recede, consider alternate styling methods, such as bangs and side sweeps. The key is to not freak out because being on a HHJ can sometimes cause paranoia to the surface. Be willing to accept whatever change that comes and adapt. For instance, I’m in the process of embracing my gray hairs. They are white to silver and sparkly as all get-out. I cannot hide them and I’m not in the mood for dyes or hennas. Initially, I hated them; then, I resented them. Now, I'm embracing them and it’s been interesting, and I still frequently stop to pick the “lint” out of my hair before I realize that it’s one of the grays.
Just ride the wave and you’ll be fine.