Utd is not a sideways step, it’s a step up. There will always be better opportunities at Utd than at Spurs.
Spurs have had their best team and best manager in 30+ years and they didn’t challenge for the league and have won nothing.
Form is temporary, class is permanent applies as much to a club as it does to players.
Look at Liverpool over the past 30 years, they were still an attractive club for players and managers to go to - aside from a couple of years under Hodgson and the 2nd Dalgliesh period (and even then, they bought Suarez).
So I agree with you, but would go further and say that Utd would definitely be a consideration for Eriksen.