For very short hauls (say 500nm) against an old 737-classic, maybe the CSeries could achieve ~ 45% improvement in fuel burn. Which is quite a lot, but it is also an unfair comparison.
On more typical missions (> 1000nm) against the new generation aircraft (MAX, Neo) the fuel advantage drops to less than 5%.
And if you consider variables other than fuel, such as the cost of introducing a second type (separate crews, maintenance, parts / supply chain, etc.), that 5% improvement in fuel may fall short to justify a purchase. Especially since the price of oil is expected to remain in the $50-$75 range through 2025 or more.
Beyond that timeline, Boeing, Airbus, etc., may have their own clean-sheet aircraft with even better economics than the CSeries. So timing, as they say, is everything.