15 to 6 is not "slight" and how would they work? 3x 17 & 3x 163? In saying that, I know that average loadings on Rochdale Road are much lower than certain other corridors - despite lower fares! Whilst single deckers might work on this corridor, the lack of capacity will play into the hands of the motoring and rail lobbies. But yes, as things stand I believe all Middleton/Rochdale Road services are designated as double-deck operated. A 10 minute service does still need planning ahead, if you are connecting onto a less frequent service. You need to identify the bus that should make the connection and then go out for the one in front! That's what I usually do.
How does the lack of capacity play into the hands of the motoring and bus lobbies? If the demand is similar after a Metrolink line opens then the capacity doesn't need to be cut. If bus usage drops then the capacity needs to be reduced. Maybe 6bph would be too greater reduction but it is reasonable alter routes to focus on areas not served by Metrolink and to switch from double to single deck buses to reflect a switch by some passengers from bus to tram? The staffing cost would remain the same but capital costs would be reduced. Metrolink lines are built on the basis that they will opperate at a profit in the long term and the cost of building the lines is loaned against the profits of the established lines and as grants from central government. Its not like trying to speed up bus journey times to encourage more use is popular either. The guided busway and the bus priority systems have received much criticism. Reopening the line to Middleton as heavy rail might please those who wish to see every change since their youth reversed but there would be fewer stations than Metrolink. On the whole I trust TfGM to do a decent job and think they will when they eventually regulate the bus network. There will always be a huge number of people whinging whatever they do, its a national hobby to help people get through their day. Some people will be pleased when a problem is solved but many will just switch immediately to grumbling about the next thing that they don't like that the government doing.