If this idea is even remotely close to what is likely to happen, Bombardier's C series probably has a huge advantage over A&B.
Unfortunately for Bombardier, the CSeries delays & budget overruns have completely changed their prospects.
As you've pointed out, the 737 MAX and A320neo are extremely successful, even on the basis of older technology.
With already more than 7,500 firm orders between them (and counting), Airbus & Boeing are quickly locking up the 150+ market for the next decade.
Even if Bombardier were to announce a CS500
today, by the time it can enter into service the single-aisle market would probably be 75-85% sold. Bombardier would have to compete in a three-way contest for scraps.
And in 10 years, when the next big purchasing cycle will start again, Airbus & Boeing may well have all-new aircraft to offer, perhaps with even newer technology than what the CSeries has today.