I tried the Cross Country train from Birmingham to Stansted before COVID, it was a horrific experience, never again. A 2 carriage train with only tiny luggage spaces, standing room only most of the way. And so slow as it meandered round the East Midlands.
I remember seeing quite a few passengers with suitcases (possibly STN bound) from Stamford onwards but I don't think the availability of this service will see any increase in Wes Mids based travellers to fly from Stansted. But I do think rail is competitive against road from Peterborough onwards travelling to Stansted.
I think if STN wants to attract more passengers to travel by rail it should talk to the train companies about a half hourly dedicated STN - Peterborough airport service which would be same platform interchange at Cambridge for any Midlands bound people plus then offering a more regular Cambridge - Peterborough service for passengers who want to connect to northbound trains at Peterborough.