Thats what makes me wonder if HRT can grow hair in 100% of the cases, i see a lot people on HRT who dont get hair results from it.
That's a good and valid observation.
Let's start with young people who have close to perfect hairlines to begin with, say from 16 to 30 but it could be older if the person has a full head of hair. From what I can tell, almost all of these people keep all of the hair that they have if they use enough estradiol. For guys with mullets, that hair seems to become smooth, grow longer and is much nicer than their "male" hair had been. This is similar to using Proscar from puberty onwards but the hair will have the much nicer female aspect from the estrogen. That seems to indicate that HRT potentially works for everyone not balding. You can see the pics on the reddit transition pic pages of men formerly with full heads of wiry hair that now is looser and free flowing and grows longer.
Let's consider cases involving HRT which I define as being dominated by a substance chemically similar to estrogen or that is metabolized into estrogen in the body. Here, actual regrowth is possible and some people grow impressive amounts of hair. I was moderately bald on top and with a fairly low but clearly degrading hair line (hair both above the forehead and along the side burns line). Virtually all men lose significant hair in front of the ears but tend not to notice. The hairline and the hair in front of the ears seem to be most difficult to regrow. The crown regrows more easily than the hairline. In females, the hairline and area in front of the ears forms an elliptical halo similar in shape to a woman's hair band.
Nevertheless, I have never seen a picture of someone with a Yul Brenner sort of hair situation, recover via HRT to the extent that it is cosmetically significant for an XY and certainly not for an XX or child-like hairline. Although I have pictures that look like that, it was due to a massive shed. My underlying condition was mostly age normal though. The good news is that sheds seem to not be linked to increasing baldness under HRT. It appears to me that the body sheds the hair to start over, jettisoning all or most of the garbage hair. I recovered twice from sheds. The first was massive and the second was a bit more subtle but both were linked to heavy used of spironolactone, up to 500 mg a day.
Still this is a long slow process. I started using over the counter estradiol but mostly estriol maybe 12 years ago and it mostly had physical effects. I added Estrogel from India but it's hard to make progress without HRT being an everyday and not sporadic process and you have to titrate the dosage upwards as you go along. Things started looking really good in 2021 and then I had the second more moderate shed. This barely fazed me having had a worse one before plus I had started preferring to wear a wig.
For a 60 year old XY, I have had an amazing hair recovery. It's definitely cosmetically significant for a male. It's still not good enough for a female yet however. The trend appears to be my friend. I have fought hair loss for 40 years without ever taking a break. I have always taken the best thing that worked first for males and then for anyone related to HRT and it still has taken me well over ten years for the bald sport to completely fill in. The hair and ear lines are continuing to improve but that's a lot of time, effort and money. Most noteworthy is that all of my hair has improved greatly in texture and manageability, and ability to grow to longer lengths.
Why doesn't HRT work for all XY's who try it? I think that head shape and size might be a huge detriment if the head has grown excessively due to testosterone. See Barry Bonds who grew four hat sizes in the early 00's and began balding immediately from a full head of hair. I also think that men with skin baldness and little to no fringe are suffering from incredible inflammation everywhere. Small amounts of regrowth actually make some bald men look worse if they are close to the Yul Brenner stage because it takes away even the neatness of being completely bald. Now you have those straggly tufts that look disheveled in the wind again. For virtually all older, more advanced cases of baldness among people transitioning, they would be well served by finding a good wig. I found one I love and now that covid is over, prices are actually dropping although we will see what the Tariffs bring. I wear them for one month at $40 per and then toss them. That's about $500 yearly for "great" female hair. Many people spend much more than that on coloring and hair cuts.
Wigs often drape and move almost perfectly and most people don't notice it's a wig. Unfortunately, male hair systems seem to be far, far more expensive and they never seem to fit quite right or drape the way that you want. Virtually all of them have the "wiry" sort of hair consistency that we saw with Ted Koppel and other newsmen and of course, George Costanza, who appeared to have a dead animal sown on his head. Nevertheless, women virtually never spot hair pieces unless you tell them.
So the OP's point is on target and we don't know the answers but in terms of seeking cosmetically significant hair regrowth for XY's with pervasive baldness, things are bleak. It's probably not worth it for any XY except those who are OCD or suicidal about hair loss. It seems to me, that if anything, baldness is increasing in general among whites and even blacks, who average 25 percent white genes in the U.S. You see far less baldness in Africa and the Far East and virtually none among Native Americans. This probably has to do with their having less T and DHT in general circulating. The joke about bald men being "real males" is kind of true. They usually have incredibly thick and long-growing and impressive beards. There's no hair loss per se really, just a complete migration downward from on top to the face. This is an instance of extreme sexual dimorphism that Africans and Asians and Native Americans tend not to experience unless they carry caucasian/Semitic genes.
