I don't think they've upped frequency, just replaced one of the flights from the RJ to the 319. Considering SN used to have usually three MAN rotations on the RJ85 during Summer and FlyBe another 3/4 rotations to BRU, it's a huge drop in a capacity that even the 319 can't fill.
What probably helped the Antwerp flight survive is that it was the last one from Manchester to Belgium. Then Ryanair introduced the Charleroi flight at the same time.