I'm not convinced there is such a huge demand for these journeys. The hourly express bus X5 from Sale to Stockport via the M56, introduced a few years ago, has been reduced to 4 return Mon-Fri peak hour journeys (2 am, 2 pm).
The bus service from Chorlton to Stockport (formerly route 16, becoming 316 in SlELNEC days) has never been especially frequent. I used to use it regularly for a year when I worked at Stepping Hill Hospital (which was then its terminus in Stockport) to travel to my late parents' house near Southern Cemetery. Metrolink (if it had existed then) would have been no potential substitute for this journey.
On 3 occasions I have parked at Parrs Wood Metrolink station to take the tram (changing at Cornbrook) to Hope Hospital for meetings, as the parking there is problematic, but it is slow, especially winding through Salford Quays. It won't be feasible to extend any Metrolink service from the proposed terminus adjacent to Stockport bus station to the railway station to enhance connectivity, as the latter is situated much higher (at the level of the viaduct).
I agree with @Xenophon that bus journeys from Altrincham to Stockport are slow and indirect. On the one occasion when I might have used it (because my Northern Rail connection from Stockport was cancelled), I investigated whether bus 11/11A would get me to Altrincham earlier, but ascertained that it would not and it would be quicker to wait for the next train. Bus 11 is useful though for travel from Altrincham to Wythenshawe Hospital, where parking is a nightmare.