The risk is that a court would have sided with the class and demanded a huge amount of monetary and/or other relief (like determining that some of Visa/MC business practices were illegal and not allowed).

A settlement allows all parties to save face, compromise, and avoid the worst possible outcome on either end. Yes, Visa/MC could prevail in their entirety and the class could get told to suck it. Or Visa/MC have major parts of how they run their relationship and billing to merchants declared unlawful. Beyond the odds of prevailing, sometimes parties get bruised along the way with ugly, ugly, ugly content (emails, presentations, memos, etc.) found during legal discovery. This is why classes that don't get summary judgment dismissal are usually settled instead of companies chancing taking them to trial.

Your watch comparison falls flat because it's pretty easy for me to go to any of dozens of watch brands, buy a watch, and then use it anywhere to tell the time. The network has to be held by the cardholder on a product AND the merchant has to understand it. It'd be like if each watchmaker had a different readout of the time (not just styling, but functionally to understand the time displayed as if we're talking English vs. Spanish vs. Chinese as languages) and you had to be able to display the time to someone else with them being able to understand it. Imagine this scenario and then imagine if 90%+ of the watches were made by Citizen and Rolex.