2x10" 40 Hz @ 100 dB SPL=3.4526
VS 4x7" the same =3.5231 Xmax.
If you say so, than yes. 7 inch drivers can have 3.5 mm excursion, no problem. In fact, in a ported box, at 40 Hz, they would hardly move at all. Ok, so do 10 inch drivers...
But that wasn't my point, that is if we look at the OP and this setup. He paid money to get a 12 inch sub, which alone (at 40 Hz) would theoretically extend itself by a whopping 1.3mm in order to produce equal SPL as 4x7. Bellow the mains tuning frequency, which is a bit higher than 35 Hz (-3db), the 7 inch drivers will start to roll off much more rapidly than a sealed sub would do. They will use room gain, but so will the sub, and then some, being a sealed one. https://www.audiosciencereview.com/forum/index.php?threads/analytical-analysis-room-gain.23211/
The lower the frequency, the mains will go more and more in excursion, until they can't handle no more. A sub will still be happy about it. So this stuff is rarely apples to apples. Perceiving low notes because of the room gain does not sound the same as having a dedicated transducer handle it, with no audible distortion.
In a sense, it can be true that we don't need subs until we hear a properly integrated one. You are right, for music we rarely need anything lower than 30 Hz. And some subs are certainly overpriced for what they are. But there is certainly audible difference, there are advantages in using them, there are people who like it flat to DC. Each to their own. It's a hobby.