I think you're going a little Kyle Fergusson on this topic, though, and projecting a top level issue onto a lower level problem. I don't dispute your reasoning, I just think you're looking at it from a scenario that isn't really as applicable in lower leagues.
It's indisputably easier to carry yourself to victory in lower tiers by doing damage (bar the occasional outlier who gets overbuffed and does well at both damage AND tanking). The tank just can't have that kind of impact on the game. While yes, a bad tank can prevent you from getting that damage in, it doesn't deny you the ability to get kills outright. Bad DPS denies you the ability to do anything at all. You simply can't even engage because it's a losing proposition.
ETC can do a dumb mosh or miss his Powerslide or whatever and it sucks, but it doesn't necessarily cost you the fight. It can, but that depends on the other 4 bodies. Typically it'd only cost you ETC. However, even if ETC hits both of those abilities, in a lower league or typical HL environment he has no guarantee his teammates are gonna follow up. He does know they will probably engage, but I can't tell you the number of times I've been a game where myself or another has hit that big mosh and not shit happened...and then it broke and they wiped the whole team because ETC set up a god engage, but the players behind him weren't good enough/ready/split targets/whatever to take advantage.
EDIT: I'd also argue tanks like ETC are the exception from their warrior brethren anyway in so far as helping their team to that degree. ETC can displace and setup easily, but the list behind that is few and far between. Most can get on targets, but setting them up for their team is a different proposition entirely. That's another reason they are hard to solo queue with, and also probably a decent part of why they nerfed ETC into the ground (3rd lowest winrate in HL now, down 8.7%) because he stood out so far compared to most other tanks.