I think, if it had filtration and a heater, and allowed water to circulate, it WOULD be better than the cups. 20 bettas in 10 gallons is still half a gallon each. Not great to live in, but better than a cup of water in a cup. heck, leave one space empty and put a heater and a filter, use mesh dividers and it would be better than the cups. Not ideal, as if one gets sick they all risk it, but that's the same in a community tank as well. Obviously with 20 in 10 gallons, it would still need a LOT of water changes. Like 2 a day. But surely it would be easier to do 1 big tank than 20 small cups.it always surprises me how LFS handle their bettas. I went to one a couple weeks ago (i just happened to walk by it and recognized that name as one of the better ones in the city) and walked in and saw their betta in divided tanks...no heater and not filter. Im talking about a 10 gallon with 20 bettas!!!!! It was divided into 10 slots and then those slots were cut in half and you could walk around the tank to you could see all the bettas. They had two of these stacked on top of each other and maybe like two rows of tanks like this. the water also looked horrible and the bettas were all sick/had major fin rot. When I alerted the store to them they said it was better then cups....
I dunno, i think with some serious tweaking, it could work.