@Bruno Schull as I wrote, I have never been racing bikes. I did race in xc skiing though. Started training at age of 6 and finished when I was 27. I have few national champion medals, I have been in national team for long time, but I never managed to qualify for Olympics or something similar, and that also means I never got medal on Olympics, so by "normal" people's standards, my career sucked. I did mostly European cup races, and even there I finished normally somewhere between 15 or 20, so yeah I never did anything good. I tried my best, and obviously that was my reach, so I don't have any bad feeling or unfulfilled dreams which would make me race now at age of 49
After I finished my racing career, I still go xc skiing a lot and I still enjoy xc skiing more then anything, but I never put start bib on again after I quit. I do this for fun nowadays and it's really good for me.
As for devices go, I have been training with HRM since late 1980s. If I remember right, I got my first Polar in 1986 or something like this, and I have HRM ever since. Nowadays, it's mostly to keep my head cool when I feel like pushing. Yeah I still do some interval trainings nowadays, but they are done without plan... when I feel like pushing, I do this. But keeping HRM on, it makes it a bit safer at my age
So for training HRM is perfect tool, I totally agree about this. HR is the thing that really tells how your body reacts to this. Sure there are few caveats and it can't be used for all possible sorts of training, but in most cases, especially for endurance sport, it works great. I might wrote slightly misleading or wrong about PMs, when I wrote they are totally useless. They are not, and in few cases they can actually work better then HRMs for training, meaning mostly for training where HRMs are useless due HR lag (short sprint VO2max intervals). And of course, if people would actually figure this our and figure it out how to do things properly, they would be most usefull device for exactly that what you wrote. Changing position, testing new/different equipment etc. Most, if not all, use them other way, similar/same as you would use HRMs, but power meters and power values for humans can't be used same was as HR data.