Medium is not dead nor is it dying. The top writers are leaving because they now have to share the wealth with a platform of new writers and they don't like it.
They do have concerns about the quality of writing on the platform and it is a concern that I share. I don't think leaving is the answer though.
A lot but not all of these top writers sold expensive classes on how to write and how to write for Medium. Once that pool dried up that's when they decided to roll out.
As for the naysayers of the "sky is falling", they're full of it. They read one worrywart and then wrote a story that someone read it and then they wrote a story until the platform became flooded with nonsense and based on nothing than rumor and inuendo.
Thus the concerns about quality here.
I've been here for nearly a year and a half which practically makes me a war vet on Medium. The platform is fine. It's the people with no skills that should be worried.