You have to understand the character and stance of DUP. They are fundamentalist bigots with a paramilitary wing that never decommissioned. They hate women, minorities and gays and have supported every despicable cause in modern history, so naturally enough, they are gung ho for a hard Brexit - something comprehensively rejected by the electorate.
EU negotiations tanking will cause ructions in moderate Tory backbenches and huge public pressure. Revitalised Corbyn will lead relentless opposition, which will be taken far more seriously by media, & local inquires and events in NI may well bring Foster down. Its a fundamentally unstable arrangement made even shakier by bizarre personalities, strong opposition and Brexit chaos. It wont last.