Like a large number of operators, Cityjet has never bought a new aircraft before, so there's no compelling reason why they should do so now. They have not long ago finished rolling over their jet fleet from a motley collection of older 146s from different sources for a nicely consistent fleet of more recent Avro RJs, which can last them for the next 10 years or so, by which time types such as the Embraer 170 should start to appear on the secondhand market.
For the prop types the F50s will probably come to the end of their time a bit sooner. There are a range of types that could replace those, I do feel the Q400 is a bit large, even secondhand, if you look at their average load factors on their prop routes.