Manchester - Heathrow sees something in the region of 75% of the users making a connection at LHR, the remaining 25% are either passengers making separate connections of their own or point to point, hence I'd suggest a Gatwick route would be a challenge for this reason and those stated in relation to the trains etc.
Point to point pax would be (at the LHR end) residents of the Thames Valley from Middlesex to Oxfordshire who don't want a long tedious journey up to London before taking the fast train to Manchester.
Probably also works in reverse with residents of Cheshire and parts of staffordshire, etc. (?).
However, only a minority of traffic is heading for city centres nowadays, and only a fraction of that is centre-to-centre. London/SE England is big enough to be separate markets in separate areas. If you live in say Maidstone and have a business meeting in Northwich, try doing that by train.
Good point!
The Manchester trains eliminated their stop at Watford, and that cut out the whole west-of-London market from them.
Yes, this is a big pain in the arse for many!