If you think you're stuck in your improvement - if you take lessons with someone who is much better than you - you will be surprised how much there is to learn.
Well. not to labour a point but "everything" isn't really a surprise

I know how much there is to learn. The thread is about whether people reach a point where they stop learning it, not whether they think they've learnt everything there is

The [welcome] surprise would be if said "good" teacher existed.
Of course, it's trivial to find someone better than us, but "good teacher" is more difficult and changes based upon the problem(s) you have improving. The teacher that's good at showing uni students the subtle nuances of interpretation in masterclasses qute often isn't going to have much idea [or motivation for that matter] to teach someone who can barely play a note where to go.
I doubt a we could name 100 people in the "good teacher" category given these specific criteria - not just "plays better" but that can actually address the specific problem(s) a student has [even if so, at 50 pupils each, that's only 5000 people that can get a good teacher, the rest have to improve in some other way - this is why books and dvds sell and why there are hundreds of people pouring over the threads in here - it's not because there's an abundance of good teachers out there teaching it all face to face and improving people's piano playing]
My sons primary school has a music assembly which is a total farce, it's just kids playing "C C C G G G" and things like that on whatever instrument they've got. If you read Bernhard's 2004 post where he talks about teaching a Chinese kid, not some prodigy, just an ordinary kid to actually play pieces, in that context you realise why that would look so impressive, because most of the kids the same age aren't being taught a $£$"£ing thing.
My son brought the sheet for 'opportunity to learn a musical instrument' home and I'm so reluctant to fill it in for those reasons, because I saw his potential frustration when I taught him to ride a bike. It was clear how upset he could get...but the key thing was he could ride the bike at the end of the day.
But I wouldn't wish upon him the total sense of failure and despair that your average piano teacher, who has absolutely no idea at all how to teach piano, might leave him with if he has lessons [not to mention the potential injuries]
Now how many of that hypothetical list live within, say, even 100 miles of one of us? In my case I'd say it's probably none, once you've factored in all the criteria, the guy that only wants to teach kids, the one that'd wants you to do a £25+ round trip every day for a 10 or 15 minute lesson before you've even found the money for his fees and stuff like that - clearly the changes of getting a good teacher are minimal - there has to be another alternative.
Most people improve irrespective of their teachers - [for the very talented, quite often because they don't even have a teacher - either because they are at the very early learning stage before ma and pa are going to go "wow!" and find him / her a teacher or because they are at the stage where they can already play and they don't really need a teacher as such to improve]
The ones that don't improve - the vast majority of people who start and get nowhere - simply stop playing and the average teachers [of which I could quite easily compete with them even though I can't play] will give you a big list of why their pupils failed to learn with things like not being interested / not talented / too lazy or whatever else to rationalise away why they didn't improve.
Sometimes of course, their reasons may be right, but not always and there aren't many teachers you'll ring out of the telephone directory that will say "Well actually, some of my pupils can play, some can't, the ones that do go on to better teachers but I don't have much idea why nor anything to offer the ones who can't <shrug>"]
The lucky ones learn at an age when their skills will find them the teacher - because any under 9yo with chops gets everyone ooohing and aahhing.
As for the subjective bit - obviously there's lots that's tangible too - I thought that would go without saying. But there's lots that's subjective. e.g just pick 2 pianists and start the imaginary thread something like "Could Horowitz have improved his playing to the point of being as good as Hamelin?" - I imagine the resulting flame war will answer the question about whether it's subjective or not
