It's about connectivity to other destinations aswell though, not just Edinburgh-Newcastle.
IMO, 3ph 'fast' is about right:
2ph LNER to London
1ph XC to give Edinburgh connections to Leeds/Sheffield and onwards to Birmingham and the south-west.
Then:
1x Scotrail all stations to Dunbar
1x TPE which runs fast to Dunbar, then connects the smaller stations enroute to Newcastle. Ideally I think this would be hourly, but 5x per day is a start.....
I agree the current hourly Northern extension to Morpeth should be merged with the TPE if this service becomes more frequent, but at 5x daily it's probably not enough on it's own at the moment.