Don't be too blinded by pure mathematics. Run a playtest where you tot up the damage inflicted up to the point where the monster hits zero hit points and THEN do the mathematics. Player tactics and synergies will make a difference in practice that you won't see on paper. You also need to compare big bad monsters versus multiple lower hit point grunts as you would expect great weapons to be more effective against giants and TWF more effective against a horde of orcs.
I do think it's wise to compare class with the same class rather than comparing across classes.
You don't want to be blind to the math either. The math is telling me that two weapon fighting is in an acceptable range for the damage it does, except when a bonus action is required elsewhere. That is enough for me to propose my house rule and test it in my game.
You are correct that ultimately it isn't how much damage you do, but how many creatures you take out and how quickly you do so. Or really, how much fun you have taking out bad guys. That's harder to gauge, but that is what play testing is for.
So far in my games I have only seen people avoid two weapon fighting, except for once. In the one case where it was used, the player had some fun with the style along with frustration with the limitations. It made him regret going with two weapon fighting. That's not good.
So my rule removes the frustration.