The problem is not should UA cancel or not. The actual issue is UA will NEVER cancel the order.
1. Even the MAX grounding covers UA's 7M9, so far there is nothing conclusive suggesting 7M9 might have the same or similar problem as 7M8.
Note - UA did not order 7M8.
2. UA uses 737s again because of the merger with CO. That's why many pilots are type certified for 737s, but not A320. If UA switches its order to Airbus, UA will have to cough off more money for training purposes. In this climate, it does not like Airbus is going to give UA a great deal.
Note - the uncertainty of UA's A350 order also impacts its bargaining power with Airbus.
3. Last but not the least, UA's commitment with Boeing actually helps both UA and Boeing, which UA can, at the minimum, bargain for more discounts with any new or existing orders, while Boeing can live on another day.