If they would want to branch out, they don't care that they will ruin a game because they are so OP that their tmm rating doesn't account for their senton skill. In fact, it would encourage them to play as everyone likes being the low rated guy that crushes higher rated people, especially at the 800-1200 rating where you are literally nobody and only recognized for what your rating is. By including sentons, you only make it more likely that these excel sheet dudes MIGHT play as at least they can feel comfortable on one of the potential maps.
And no, I disagree entirely that some sentoner will be 500 on many of the 20x20s and 1000 on sentons. And again, wonder wouldn't be in a full share pool, it would be share until death. Huge Wonder would be full share, but that's because it's a hugely spread out land based 20x20 with tons of tree reclaim and massive rock buildup in mid to express control over.
You know to reclaim trees? You know to make an air factory? You know to drop sometimes? You know to hit mex upgrade button? Congrats, you're 1000 on every 20x20. Only thing that is different between Huge Wonder and sentons is that you need to actually learn about units expressing map control in order to keep your eco safe since I imagine most low level sentons don't involve drops or t2 air harassing eco and instead is two dudes massing up navy to smash into each other.