For the officers the treatment wouldn't have been that bad. But for the commoners the treatment would have been awful. They would either have been chained and put in a cell while on "starvation rations" or had to work on ships as slaves. Which wasn't any better, maybe even worse.
The Spanish would have tried to force them to convert to catholicism. If the prisoners would have converted the treatment would have been much better. But even in the real Wager situation some people who ended up as prisoners of the Spanish refused to convert and endured the terrible treatment.
The geography involved meant they had little choice to change their mind. They could have made the decision at the start of the journey and sail north but once they were on the way, there was no option. There were no Spanish settlements on the way. The nearest one would have been close to Buenos Aires but at that point you are just little ways off from Brazil and you might as well push on.
The only people living down that far south were some nomadic tribes. They would either kill them outright or enslave them. Which happened to some of the people left behind. I totally understand not wanting to gamble on that either. And there wasn't even a guarantee they would be able to make contact. So they had to push on.