The difference is that the snippet points to the source. If you are like me, when I see the snippet and find it relevant I immediately go to the source, since I get access to the full context and others' comments. With a technology like chatGPT, that link is cut.
But if the traffic to SO is reduced significantly because of Google, there will be a death spiral. There wouldn't be a site to go and ask new questions, and even if the site is still there, nobody will bother to answer. The core problem is that the people who create the new knowledge (the answer writers) and the people who aggregate the knowledge (StackOverflow) will not get any concrete return for their efforts, since they are cut out of the consumption loop.
Here's a concrete scenario:
StackOverflow makes money because people go to it for answers to their questions. The answers are provided by people who write them because they get points for providing good answers, that they use for reputation points to get good jobs.
It's an ecosystem. Everybody profits.
Now Google scrapes the contents, and provides a service where you just ask your question and it gives the best, most accurate answer. Nobody goes to StackOverflow any more. People stop posting answers, because nobody asks the questions, and nobody reads their answers any more. StackOverlow goes under, and the ecosystem dies.
Where does Google gets its information now? The feeder pipeline has died.