Realistically, you're right - fare cuts just don't happen.
Although I accept the service will be better once the trains are refurbished, the current 'cheapest fare' Wellingborough to London is £47.50. that's £95 for a round trip. Wellingborough is about 65 miles from London by rail.
One alternative would be to go by car, bus or taxi to Northampton, where the similar fare is £18.60 single, saving £57.80. (and more trains than even the new Wellingborough service)
But has an every 30 minute (rather than 2 trains per hour) service from St Pancras been promised? Providing an even interval service for Northamptonshire has always been said to be impossible, due to platform occupancy at St Pancras, in the past.
I hope they don't find out that the 360s are in a worse state than they expected, having been neglected by their previous operator, and don't all get done. That has happened, I've heard somewhere...