Such a service would suffer from the similar issue that nobody would ever ride it from one end to the other. I suspect Westbury to MK may be faster via London than via such a local stopper. If there is any westward EWR extension, it should probably run fast to either Reading or Bristol.
Westbury - Swindon 42 minutes
Swindon - Didcot 18 mintues (my estimate for 100mph diesel stock)
Didcot - Oxford 19 minutes
Oxford - Milton Keynes 44 minutes
So about 135 minutes should be possible.
A 7 minute connection at Reading produces an 81 minute Westbury to Oxford journey time at the moment which is slightly faster for that section of the journey, but there would then be the connection time at Oxford for onward travel towards Milton Keynes.
The journey via London between Westbury and Milton Keynes is longer than my estimate for my hypothetical service.
There is also all the intermediate journeys and connections that such a service would provide, plus freeing up some capacity on London services.
But hey, this speculation is just a bit of fun.