Nobody knows what will happen at the moment, we will only find out when the line opens. I hope it’s an outstanding success considering the amount of money being spent. I would like to think a lot of motorists will convert to the train which will help with congestion. I’m not sure you will get many people catching a bus to Ashington bus station then walking to the station to catch a train, unless an interchange is being built. Then people will consider where they want to be in Newcastle, Haymarket or Central? 
Reducing bus services will not help passengers travelling locally in Ashington or even those catching a train to Ashington then missing their onward bus. It will be an interesting time when it opens. 
I travelled on the Okehampton to Exeter line last year and was very impressed. All trains were busy, however I’m not sure what the buses to Exeter were like  before that and all dedicated connecting minibuses from the station didn’t seem to carry many people.