Southport to the airport has been discussed many times before. I'm not sure anyone has ever showed that there is sufficient demand for an a direct airport service from Southport, opposed to the former Southport to airport service existing for operational convenience and being used by some people.
Most Sandgrounders and West Lancastrians would choose Liverpool airport - cheaper flights and far less of a long-term issue with security queues. And they'd likely drive or take a taxi due to the typically antisocial times at which low cost carriers depart from and arrive back to their bases. I would agree the
Airport is not an important destination for Southport. Castlefield is - most significantly Oxford Road which is a fair way from the tram* - but not the Airport.
It's also not, to be fair, a particularly important destination for the TPE Scottish services either. If you wanted to go to Edinburgh or Glasgow you'd not fly to Manchester. The Lakes is maybe a side-trade there, but many would just go for the direct Northern services to Windermere even if it involved a bit of waiting around.
I do see the point about Stockport, but with such a frequent connecting service I think it's far less of an issue than Southport-Castlefield. As I've said I wouldn't object to half hourly to Vic
if the connections were good at Salford Crescent/Bolton (ideally the former). I don't however trust that they will be.
* I think I'd withdraw my objections to "all Southports to Vic" if the tram operated down Oxford Road to the uni, as it should. But that's for another thread.