I've always thought a TF2 ProMode would be enjoyable to at least play in the case that Valve never delivers. In terms of making the ProMode the official comp TF2 game that everyone would play, I don't think it'd work and be very hard, but as a side game for whoever wants to play in it for fun(and get away from all the bullshit), it'd be cool to test around. Perhaps it might even inspire Valve(lol) to copy over some changes.
To be honest, I find it ridiculous that Valve hasn't implemented at least some basic aspects of comp TF2 6's to comp MM yet and the restrictions they put on comp MM, particularly the class limits, viewmodel restriction, HUD restriction, and even graphical restriction. One of the few many spoken reasons about that during that time was so people wouldn't be confused when they watch a player's stream and see different stuff, or more specifically a high level player. It would seem that if Valve was trying to treat the game like CSGO or try to make it in a way for the future of Valve's vision of "competitive TF2" in eSports. But they obviously don't care enough it seems, and they have yet to truly deliver upon it or put an effort towards it. Overall, it's just a waste that comp MM ended up being when the comp TF2 community has developed over many years and solidified what works and how it should be, but Valve just chooses to just ruin all hope.
We can wait longer, we can continue to do what we're doing, we can deal with and live with the issues that Valve does not want to fix or cooperate with for many years more to come(and have already), what's another year or two...or three or more. A TF2 ProMode isn't out of the question completely, if it means for a chance to finally play the game how it's supposed to be, make the changes that we want, and possibly create something better.