I would hope that City Airport would ensure they have a good mix of traffic. A wider spread of routes would benefit them more longer term than a third "me-too" operator on Zurich, for example, has done.
Jersey-London is a good business route mornings and evenings (look at all those banks, solicitors, etc in St Helier), with tourist traffic in the middle of the day. It also, compared to other UK domestic destinations like Newcastle, has a high proportion of O&D traffic rather than less-yielding connectors, but there are sufficient of those as well to other CityJet destinations to occupy some empty seats. VLM's once-daily off-peak operation, although just a fill-in from their mainstream business routes, does not meet this nee at all; I am surprised it has lasted as long as it has.
I would encourage Scot/CtyJet to go for 3xdaily Jersey, based from there. I'd also suggest that CityJet use them for some of their lower-load midday flights rather than an RJ. You would have to keep this to the non-competitive routes.