In addition, you do realized that the opfor is not going to consist exclusively of Abrams, Leo IIs and Challengers etc right?
100mm with modern HEAT or sabots will ruin any Bradley/Warrior/Stryker's day as thoroughly as any 120/125mm.
Besides, even if the 100mm can't outright killed an enemy MTB, it does not mean that any Abrams that takes a 100mm shell in the face is going to be laughing it off. At a minimum the impact is going to shake the crew for a few seconds/minutes. That could be all the time it needs for your own MBT to pop off the first shot as opposed to the Abrams.
It is a classic mistake to compare only paper values. In WWII, the Russian T34 or American Sherman's main guns could not realiably penetrate the German Tiger's front armor, but they still killed plenty of Tigers all the same.
At the end of the day, if I was in a city fight, I would rather have 100mm guns on my side than no big guns. And if you are fighting enemy infantry, 100mm is as effective as 120/125mm.