See below.
If Brexit happens the EU will be severely weakened, possibly terminally. They will be looking to lose poor nations not acquire them, both Scotland and Wales will soon be counting their current blessings. As prospective small member states, separated from the EU by their primary market, their economies could never survive unsupported in the trading environment that would follow our departure. Precedents just don't come into it, economic interest will drive all the decisions and England holds all the cards in that context. As I said, the NI is different but this deal gives the Unionists their best chance of remaining in the UK.