The problem for TfGM is that they had the right idea in the 1970s with a cross rail type scheme that electrified all commuter lines with an underground section through the city centre with new central stations. It had the potential to deliver a frequent high capacity multi carriage trains that increasingly with the massive growth of the Manchester city region we now need. This was rejected by the then government as too ambitious with a backdrop of industrial decline and falling rail use. TfGM went back to the drawing board and came up with a lesser capacity, slower and much cheaper solution, Metrolink. They did the best that government would allow for Manchester at the time.
In simple terms the strategy that they recently produced and subsequently Stockport builds on the 1980's thinking. I believe it is wrong considering that rail growth is predicted to increase in the coming decades. Existing rail lines into Manchester should never be considered for Metrolink tram-trains, they simply will not cope with the predicted demand and on routes such as Marple/Rose Hill will be running at much lower frequencies to allow for normal rail traffic, I believe that they will be slower than the existing service. What is good about Metrolink is that it is better than the bus, so routes to Eccles, Ashton etc. open up new rail corridors that would never be viable using heavy rail. This is the case with Marple to Stockport, however, the business case will be difficult if not almost impossible to justify.