Isn't this DUP deal supposed to be only for two years or something? That suggests it's either up for renegotiation, or the entire government self destructs right after Brexit.
The only realistic way to bring them down is five Tory by-elections. That would wipe out whatever working majority the coalition has. The odds of sufficient numbers of Tory MPs turning on the government is pretty low, although not non-existent.
I suppose it's possible that the DUP could ask Labour what it's worth to help bring down the Tories, but that would probably just be a tactic to force concessions from May.