Because the goal is to not to "deal" with the carpet of doom, people are asking to remove the carpet of doom.
The human can already deal with AI hordes are high levels, which is why we have hordes to begin with. Adding splash would just make the horde easier to deal with....which defeats the purpose. The real ask is to provide equal or even greater challenge with fewer units, and that's why its such a tricky problem. Put another way, what people are "really" asking for, is a fundamental change to Civ 5 warring, to make it where the humans "immortal unit" play no longer lets them beat armies 2,3,4 times their size....so that we can remove the armies 2,3,4x the human's size.
But it also goes back to a fundamental question taht has been somewhat brushed aside.....does the AI actually have "hordes" now? Sure a few versions ago, it was carpets of units, but the AI has gotten a lot of key nerfs since then, the biggest one is no ABC production bonuses. That's a big deal, and will definately mean less units for the AI. So is it enough, is the AI really providing carpets of units still, or is that just people's old expectation and the problem has already been addressed?