For WestJet and Southwest as it expands beyond the continental US, the MAX 7 will fulfil a role that both the other MAXs and the C-Series can’t fulfill due to range and runway limitations, they would need to go to the A321neo or the LR to get a greater range, but they would serve different narrow body markets due to the significantly higher seating capacity.

I can’t see Southwest not staying with Boeing and WestJet will need to change it’s business model somewhat around niche narrow body markets such as serving some Caribbean destinations with short runways or smaller Canada to Europe or Hawaii routes too small for the 787 but too far for the MAX 8 or 10 at full capacity, such as YYZ-LIS, YHZ-FCO or YWG-OGG.