There are not many of those "...holes gouged through greater Manchester for un-built motorways which would be ideal for street-level tramways,...", the only one that comes to mind that is left is the M60 /Hazel Grove by-pass. I doubt Hazel Grove will be impressed if that became a tramway rather than a badly needed road.
I'm sure you'll correct me with the ones I've forgotten.
As a regular user of Metrolink, although it is not perfect it is far superior to the heavy rail that preceded it.
I had in mind the corridors of empty and underused land surrounding a number of main roads through inner city Manchester eg Regent, Rochdale, Oldham, Hyde, Princes, Stockport and Chester Roads.
It was not a binary choice between an endlessly run down heavy rail operation and light rail. Manchester could and should have had a proper heavy rail suburban network. As an irregular user of Metrolink, I'd prefer something faster, more comfortable and more capacious