Most people don't want to go *to* Picc, or much less Vic. They want to go to Manchester City Centre, which the tram makes a far better job of.
Precisely!
Something that's easily forgotten is that everyday passengers aren't making the kind of journeys that enthusiasts do - more people from Atherton/ Glossop/ Marple want to head to the offices/ shops/ bars of Manchester than visit a mainline train station as their destination.
If you can provide people with more frequent services that actually run into the heart of the city (and do so with an operation that requires no operational subsidy, unlike Northern Rail) then surely that's a good thing?
Look at the Oldham line - when the four trains per hour were replaced by five trams per hour (initially), passenger numbers rose significantly - because the number of people wanting to get closer to places like the Arndale vastly outnumbered those wanting through tickets to National Rail destinations. Not perfect - through-ticketing to/from Metrolink isn't as good as it should be - but the evidence seems to show that passengers prefer five trains per hour (into the heart of Manchester) than four trains per hour (to Victoria).
Plus, given well documented capacity problems at central Manchester stations, converting the Atherton/ Glossop/ Marple lines to Metrolink (or "TramTrain') would free up some much needed capacity!