Well removing the SNP and Labour from the elections would make a Labour comeback a Very difficult hill to climb. Yes he would lose some 12 Tory seats, but the remainder of the seats are against him.
However he's already on the hook for the partial NI separation, he might not want to get the reputation for being the destroyer of the UK. It is hardly fair because that was Blair and when he created the devolved assemblies it was inevitable that the SNP would eventually gain control of Scotland and Cameron went for the Brexit vote.
If you look at it from the other side of the border, a Scottish government puts the money where the Scots want it. No more massive upgrades to the M74 and the west coast links, the A1 is hugely upgraded to the border, the A9 is, finally, getting the dualling that has been needed all my lifetime and the Aberdeen bypass is going in at long last. These are infrastructure projects that Westminster has no interest in but are important to a whole raft of Scots.
When all that money starts flowing in a different direction it makes quite a compelling story for the SNP and people who just accepted things as being "the way things are" suddenly ask "why it has taken 50 years to get what we needed".
So Boris gets the poisoned chalice, the extra boost in votes and the reputation as a UK wrecker. Quite an achievement for someone who had nothing to do with any of them....
Yes, I know, he asked for it and he will get it. It is one of the reasons nobody really wanted the job, nobody could see a way out of it with a whole political skin and Boris, after all, is hardly a career politician.