Sp**s will always be Sp**s. They'll never be as good as Arsenal, even if they finish ahead for a couple of seasons. I mean, they were due after 20 years in a row. Arsenal will be back ahead of them in short order. United has really been more of a rival on the pitch for decades. If they get knocked out of the top 4 and the CL, it could have more long-term consequences and allow Arsenal to get the upper hand on them again. Plus, Mourinho is an ass. Seeing him fail is more important than seeing any club fail.
Mourinho is an ass, but he's a washed up old ass. He'll be his own downfall himself. Besides, if Utd fail spectacularly this season they'll just keep on throwing money at it till they get it right. They have too much money to truly fall away. You can't get rid of Utd, but you can get rid of Sp*rs.
Watching Sp*rs' best team in decades disintegrate before it's even done anything would be far sweeter than watching a juggernaut find more excuses to break transfer records.