It's a sunk cost. All the time and effort we spent propping them up could be use to build a new but people won't face the fact. They think they can slowly steer it in the right direction but all the effort that has to go in is easily reversed by some insignificant amount of money from the doner class.
You're almost right. The stable situation under FPTP is two big parties. But if one collapses, it won't necessarily be revived, it can also be replaced. There might be a period where the party on the way out drains votes from the newcomer, but things can change quickly as well.
All this is moot. There is no party that will be able to wrest power away from the GOP come January. Yet still there will be fools putting all their hopes in the effectively powerless DNC.
Consider the possibility that resistance against fascism (and other totalitarians) never happens through established parties. The fascists are like termites: the edifice already has to be rotten before they can get established. Where dictators and illegitimate regimes are forced out, it has involved people getting out in the street. Established parties never lead, they only follow.