It's not about travel to city centres, it's also about travel through city centres to other destinations. Considering the goals of reducing pollution/congestion in the region, we want more people to be able to use the rail systems to get around.
Frankly, noting that the southern suburban lines can be converted to Metrolink is beyond the point. They are simply a convenient place to through-run to from the North, with obvious added benefits for those communities. Areas like Salford, Bolton, Wigan in Manchester, alongside Calder Valley, Huddersfield need better connections to the destinations Piccadilly provides access to. That's areas in the South of Manchester (e.g. The Airport, Stockport), but also destinations including the East/West Midlands, Wales, South East/West, Northern WCML (scotland, cumbria) and Warrington (all only acessible through Piccadilly or Castlefield Stations).
So considering all of this, that these communities are desperate for acess to these services, you can see not only why Castlefield has become so congested, but why a tunnel under the city, moving some local services under the city, providing direct connections for Salford, Bolton, Wigan, with the option to convert the Bury line into a more metro style service (something that could be useful considering how high ridership is), then it seems rather sensible. This would provide more capacity to facilitate light rail movements across the city centre (Metrolink is already planning a tunnel to deal with heavy traffic of the lines crossing the city centre) for any new routes that may be added.
Perhaps moving some of the WCML north services to Victoria could help (if capacity was increased there), but that still doesn't resolve all the other destinations Piccadilly provides acess to. Maybe completing the Northern Hub could help make services reliable again and help close the gap between Manchester's stations, but I'm skeptical that anything besides building a proper heavy rail solution to the problem will actually go towards resolving anything.