Isn’t it quite self-entitled to walk into a space deemed as quiet and expect to make noise ad-infinitum? I never really get why it’s so difficult for some people to keep it zipped for a relatively short period of time.
We are all different and as has been stated so many times in this thread, the TOCs are not consistent with their 'rules' for any quieter areas. Add to that the different posters' opinions of what a 'quiet' area is meant to be (e.g. no phone use/games/tv, no general talking, no coughing, - maybe no any noise including breathing in an anti-social way, then there is no majority view.
Of course, the majority of people have a strategy for dealing with this, and that’s to simply travel by car.
I don't think that the amount of noise that other passengers make is the major deterrent, - indeed on these very forums so many aspects of rail travel have been cited as major reasons why footfall is lower, (high fares, poor reliability, dirty, inadequate service, wrong type of seats, poor air conditioning, rough ride characteristics, surly or antagonistic staff), - and that's before the rather more niche aspects of enthusiasts are claimed, - too noisy trains*, lighting too bright, wrong livery etc..
Having said that, you’re probably right that it isn’t worth getting too bothered about as it’s not going to change. Indeed standards of public behaviour in this country only ever seem to head in one direction, and it isn’t upwards.
Those who seriously believe that German/Japanese/Swiss etc., railways are so much better than the UK's offering need to weigh up all aspects of living in those countries, i.e. not just how the railways work. If on balance other countries really seem better than the UK, we are fortunate that UK citizens have a lot less trouble migrating out than others currently do coming here, so the solution is in their hands.
* Suggestions here that class 150s don't have a quiet coach reads like a joke. The thrash of the engines and other general din often drowns out even the most boisterouis of passenger groups so any semblence of 'quiet' would still be absent with no passengers.