And yet you seem determined to convince people of your position.
You proposed a change because the Champion isn't working for you. People told you why your changes might be problematic. You can either reject or accept that advice and move on. You seem to be taking their criticisms a bit personally. You shouldn't. A few months ago I proposed a change to the stoneskin spell and got a similar response. I don't believe I reacted by taking it personally and becoming argumentative. People made criticisms, I argued my position and eventually decided against the change.
That you have gotten through 8 levels of play with only one attack having advantage is a huge read flag for me. Something is off. I'm not saying your game/group/style is badwrongfun...to each his own. But advantage seems to me to be a core mechanic to the game and the tactician/optimizer in me sees that as the focal point of the champion class. Find ways to get more attack rolls and you will get more chances to crit. Action Surge+Two Weapon Fighting+Advantage = 14 chances at a crit at 11th level. 1-.9^14 is 77% for at least one crit and probably better than even odds at 2 or more. If that doesn't sound like fun to you then, yeah...you either shouldn't play a champion (again...what's wrong with just using 4d8 superiority dice to do extra damage when you want instead of relying on crits?) or go with a straight damage bonus. I'd say +1 at 3rd and +2 at 17th and maybe make crits knock down or push foes. +Prof bonus seems too high to me. 10 hits and you do the same extra damage as BM does with his superiority dice with no need for resource management. That seems a bit over the top to me. YMMV.
As a DM, if a player came to me and said they are finding the champion class to be not living up to expectations the first thing I'd do before modifying the class is tell them and the party to try to give the champion advantage more often. If the player said he wasn't interested in that, I'd hand him 4d8 and tell him all about superiority dice. No advantage necessary.
The greatest thing I've learned so far in this thread is Remarkable Athlete is pretty ho-hum. As a DM I'd just let it stack with skill training but probably not expertise or similar.