I agree with everything you said. The people that wanna improve will do so with or without the parser, even though the parser is gonna be a very precious tool for this. The people that don't, well you can usually spot those even without a parser, their performance just screams "I'm only here to get carried" from miles.
And yes, I do see the need for it in EX and Savage content and as I said I'd welcome it there(maybe I didn't phrase it quite right, sorry, it's still early xD ). But imo, stuff like drex and nm trials shouldn't be subjected to this because even subpar performances are enough to clear. Would I like to do a drex in 15mins instead of 25? Sure! Personally my performance can support this and is always nice to end up in a group that performs the same way. But is it something I expect from everyone? No, and it won't ever be. It's content designed for everyone whether they have the drive and time to optimise their performance or can only log for 1-2 hours a day and barely manage to farm half of they stuff they want. A personal parser would be more than enough in that kind of content if all we care about is improving ourselves.
And yes, ofc a tool is not to be blamed for it's usage but unfortunately I don't have enough faith in this community to support the unconditional access to what I believe will become a tool of harassment instead of a tool of improvement. There are already numerous "incidents" posted here and there about people getting bashed for subpar performances in all kinds of content and even though there's no way to know if their performance was indeed bad or not there's one thing there that is indisputable. The way they were treated is horrendous. Even if they were performing badly there's numerous ways to say something but, at least what reaches the forums, is cases where they were being bashed and shamed right away, not polite comments of trying to mix up their rotations, no helpful advice on their skill usage, no encouragement to experiment further.
I wish I had the kind of faith that you seem to have in the general population but I don't. That's why imo the most sensible approach is a personal parser that can be hidden in the HUD options, with the ability to enable a party parser in the party options when you go in somewhere with a premade(which is how you go in Ex and savage anyway, who is blindly queing for them? Especially with raid finder on it's way), the same way min ilvl or undersized are enabled. That way people that are raiding have an invaluable tool in their disposal while the general population is not subjected to a forced feature that they do not wish to have. I think that's the best compromise to please both sides.
tl;dr: people that wanna improve will improve regardless so give parser option in party settings for premades to have this invaluable tool and don't shove it down everyone's throats.