And most importantly, Airbus brings manufacturing capacity (in the US too, which now might turn out to be useful).
Airbus is not bringing manufacturing capacity as part of this deal. Bombardier is on the hook for building a brand new facility for the CSeries in Alabama, at an estimated cost of US$300 million.
So why did they embark on such a programm? Totally underestimated the cost and then threw more good money after bad?
Recall when the CSeries was launched, the price of oil was nearly $150 per barrel (Brent crude) with prominent Canadian economists projecting $200/bbl oil. The program's entire business case was predicated on high oil prices. But today oil is "only" in the mid $60s and will likely to stay within +/- $10 for years to come -- even with political instability in Saudi Arabia and fresh OPEC production cuts.
With today's relatively low oil prices, it's hard to justify spending $10m premium for each CSeries compared to its less fuel-eficient alternatives, unless an airline can get steep price concessions from Bombardier/Airbus, receive government subsidies, or can operate in niche markets.