pax britannica
As far as I know a contract has already been signed for a new regular sea freight service for the time after the decomission of the RMS.
A small amount of air cargo might come handy in urgent cases but air access is supposed to be a passenger service in first place.
With 120 seats the CS100 is smaller than the Boeing 737-800 but clearly larger than an Embraer. What matters is the short field capability of the CSeries. And it is a new modern aircraft with 20% lower fuel burn and 15% overall lower maintenance cost. If the CS100 can handle the windy conditions - it seems to me - this aircraft has the potential to attract and bring tourists.
C Series - Commercial aircraft - Bombardier
I'm not even sure if smaller Embraer or CRJ have the range even for Walvis Bay-St. Helena plus Ascension in case of deviation.
And they need to be ETOPS certified also.