A mass exodus doesn't really happen in the traditional sense unless shit really hits the fan. For that to happen a large majority or even everyone has to be displaced at once and there can be no way to salvage the situation. In this case, there were a lot of short term ways out here for users not directly affected.

But, the whole situation is more akin to a war of attrition. The ones not convinced by the big things, will be convinced by the smaller things that accumulate over time. Goodwill for reddit is at an all time low, which hampers their ability to grow since word of mouth is effectively dead. People that provided effective labour for reddit in the form of moderation or content aggregation lost their morale to continue. Not all of them for sure, but it might very well be a critical mass (even if they didn't move to lemmy).

It's like a line of dominos increasing in size, if the ones that fell now were big enough to topple the next, eventually there will be a ripple effect. Eventually the quality of content goes down, the discourse turns stale and antagonistic, and communities fall apart. Only once the users who took the easy way out now realize that will they finally start the process of moving. And if reddit was doing so bad they had to make this move, I can only assume their future will be very grim indeed. The seed of destruction has been planted. (And if you want an example of that future, look at Twitter)

Whether or not that all actually happens, I'm not sure. I'd like to believe it will, but some people revel in their unreasonableness, and they're often the easiest to exploit for financial gain. I think the best thing is to stop looking back, and focus on what we have here and now. I think what lemmy has achieved so far is already more valuable than reddit had.