Well, first off, it wont get passed, so the whole debate is pretty moot, I think.
If he did get one passed through this parliament though: you say that there's no reason for a Brexit Party after a deal is passed and the UK is out; by the same token: what reason is there for voting for Johnson and his chaos after acheiveing one? Why would LP voters he needs to get onside then vote for him (the only way of getting a majority for the Tories) when they'd only consider backing him to end the impasse on Brexit?
Also, if he got a Brexit deal passed on May's terms, the Tories would be still struggling to contain the damage from a party like Farage's (and Farage is THE most effective politician of his day) and their complaints about still being effectiveley tied to the EU for years and years (probably indefinitely). I wouldn't ignore what happened in the Euro elections a few months back. Even if the Brexit party had half the impact they had then they'd crater the Tories in many of their own safe constituencies. Farage doesn't care about the Tories and their objectives, he cares about Farage and his.