Inter-suburban traffic is generally too thin and scattered, without any major traffic destinations, to warrant new rail lines, even light rail ones. New Metrolink lines are only justified if they provide additional suburbs and satellite towns within Greater Manchester with a direct rail link to Manchester city centre.
Even if the East Didsbury line is extended to Stockport, the quickest way to travel from Stockport to key destinations beyond Cornbrook (such as the Trafford Centre, Media City and the Old Trafford cricket and football grounds) will be to take a heavy rail service to Piccadilly (or ideally Deansgate) and change to Metrolink there. In my opinion, extending the East Didsbury line to Stockport, which is not straightforward as the previous alignment to Tiviot Dale has been destroyed, will have a poor business case and is being promoted primarily for political reasons.