Intriguing.... Time will tell if it starts and then lasts...
Cheapest return LCY - ANR is around £110-125. So a €70 return would definitely entice some of the financially canny passengers away from LCY, particularly as many originate from north and east London.
The Cityjet F50 3x daily service is generally well patronised, so I'd say there is demand. As I've said before, Antwerp involves a change of train in Brussels with an end to end time of about 3hrs, so a decent flight to/from small airports with minimal check in times and flight time of around 50mins can still compete.
Anyway, we'll see what happens.....