What are the loadings like north of Edinburgh on the Aberdeen and Inverness services?
Quite heavy up to Dundee, and light north of that. Dundee-Aberdeen is half hourly + NXEC and XC services, so that deals with the traffic quite well. But Glasgow-Dundee is only hourly at the moment, and the trains can get quite busy, especially with Perth and Stirling passengers. Edinburgh services were worse still, but there's now a half-hourly to Dundee which takes a lot of the strain off the turbostars.
Aberdeen passengers do, however, want "intercity class stock". They believe that the 170s are unsuitable for the journey. I disagree, because I think they're simply comparing FSR to NXEC, and NXEC's service
should be superior as it goes on to London usually (let's not go there, though!).
That said, I think there would be demand for this. More people are travelling up that way and choosing this over their cars. I would say that, in 10-15 years time, there would be demand for a full intercity service, with a proper buffet and perhaps restaurant service on some peak time trains. The allows the 170s to be put on the interurban routes, which they will do a good job running.
Of course, this is a very long way into the future! I definitely do not see an urgent need to replace the 170s at the moment, and the proposed hourly Glasgow-Arbroath service, if it comes to fruition, should deal with passenger rises from Perth.
EDIT: I was thinking about Transport Scotland's possible order. NXEC north of Edinburgh could certainly cope with 4-5 cars; for the moment, anyway.