I would say that Manchester is a spoke rather than a hub! While it chases the likes of Ryanair, easyJet, Jet2 et al it will never became alternative to Heathrow.
The airport is already almost full to capacity at peak times so how would they expect to "accommodate" any overflow from Heathrow?
However, the truth is that any "overflow" would be directed to Dublin or Madrid by IAG and to Frankfurt, Amsterdam and Paris by the other alliances.