I think it probably won't cause the site to collapse now, but will ultimately be a major contributor to it doing so later. The biggest problem for many people leaving reddit is that the various alternatives all lack the most important part of a social media platform: all the users creating new content to consume. Many people are okay with moving to tiny platforms of course, the growth of Lemmy in the past days is an example, but quite a few people on Reddit are not going to be, missing more niche communities or just not knowing much about these underpopulated places. Of those people that do leave, or move to using a new platform alongside reddit, many will probably end up eventually giving in an going back. But some people really will stay, and the alternatives will grow massively compared to their humble origins as a result. Some might die out and fail after awhile, but the ones that don't will become small but viable communies that have enough content and users to at least be usable, if not as developed as they could be.

Inevitably, reddit will, someday, do something to anger it's userbase again. But that time, or a few cycles of this down the line, when people start looking to alternatives, one or more will actually be big enough for the majority of redditors to actually be willing to move there. At that point, the site may collapse, the damage reddit does to itself with this incident will become apparent: motivating people to build it's own competition.