Where would they move to? It's not like the USA, where there are a lot of really large metropolitan areas with not professional team (hell, even Los Angeles went years without a team). It seems like every neighborhood in London has their own team (some large, some small). To move a team like Manchester United would mean losing all of the local fan base and a huge majority of the out of town supporters. Add to it that there are no large cities in England pining for a professional team of their own, I just don't see it.
If the Glazer's ambition of Relegation Immunity and a Super League Closed system are realized then you essentially only have 12 permanent teams in Europe's highest division.
Manchester (United, City)
Liverpool
London (Chelsea, Tottenham, Arsenal)
Barcelona
Madrid (Atletico Madrid, Real Madrid)
Milan (AC Milan, Inter Milan)
Turin (Juventus)
There will be a high temptation for a parasitic American owner to move their franchise from a 2 or 3 team town to a new one team city that will fund their every whim and build them a new stadium.
Large cities like Rome, Marseille, Istanbul, Amsterdam, Edinburgh, Brussels or mega rich cities like Oslo, Geneva could easily pull a Super League team whose home city didn't meet their demands.
A cold blooded creature like the American Kroenke would have no problem moving Arsenal to Rome or Oslo. Afterall he sold out his own home town and home state to move the St. Louis Rams to Los Angeles.
This is the plan of the greedy ringmaster, Joel Glazer and his compatriot Stan Kroenke.