People outside of Leeds and Bradford are just not interested in the slog to get there and a poxy mile long link road will change very little.
Unscientific analysis using google maps and going by road to Church Fenton and LBA with MAN thrown in
Sheffield: to Church Fenton: 1h 2m. to LBA 1h 12m. to MAN 1h 15m
York: to Church Fenton 30m. to LBA 50m. to MAN 1h 31m
Scarborough: to Church Fenton 1h 18m to LBA 1h 41m. to MAN 2h 20m
Scunthorpe: to Church Fenton 59m. to LBA 1h 15m. to MAN 1h 49m
Doncaster: to Church Fenton 43m. to LBA 1h 4m. to MAN 1h 27m
Hull: to Church Fenton 1h 4m. to LBA 1h 25m. to MAN 1h 54m
Barnsley: to Church Fenton 51m. to LBA 54m. to MAN 1h 6m
Some "slog" when we're talking ease of access to Church Fenton instead of LBA are seeing journey times saved venture from under 5 minutes to 23 minutes.
Looking at the Google Map image, I wouldn't exactly say it's going be easy to expand the runways (remember, you want a "proper" airport - whatever that means - in place of LBA so a minimum longer runways to have "proper" planes to "proper" destinations).
Then this assumption that 4 million more passengers will magically appear out of thin air because there's a "proper" Yorkshire airport. Can we have a list of these airlines and destinations that would spring up? MAN has single daily services to a handful of US destinations and MAN is serving the whole North of England. Airlines aren't going to cannibalise what may be a borderline route by adding a similar service over the Pennines so they can "enjoy" 2 loss making routes for a while before either waving bye-bye to the North of England in it's entirely or retrenching to back to MAN.