No, that’s not likely to have been the outcome without a union in place. The more likely outcome would have been a policy within their contract requiring the pilot to work in the case of a holiday period crunch, if the pilot were lucky enough to have any contract at all.

The 150% maximum pay was also likely a negotiation point between the airline and the union when agreeing on a contract.

You seem to think that negotiations between an at will pilot and an airline are on equal ground, they are not. The pilot is very unlikely to get a better deal outside of collective bargaining. That is why people are willing to pay a due and give up some freedoms in order to use them as leverage in compensation negotiations.