@enderfall people leave because of the state of the meta every month - everyone likes different metagames. I think this b&r season was quieter than usual. While there were people upset with the state of things on TMD, it really wasn't more than usual. I wouldn't assume there's a consensus on what a healthy metagame looks like - threads like this should only drive that point home.
This is a cop out, which a number of players have tried to take. Ongoing changes doesn't mean there's no trend. Players leaving the format is compatible with a growing, healthy format. Players leaving the format is also compatible with a shrinking, unhealthy format. Without other signals, we should interpret it as a weak signal of shrinking (because if you lose 3 players all else equal the format is 3 players smaller). With corroboration from (e.g.) reduced MTGO Daily Event and Power 9 Challenge attendance we should interpret it a as a moderate or strong signal that the format is shrinking and unhealthy.
I think that interpreting Vintage participation on MTGO as anything other than a failing of MTGO development and management is a huge mistake. Paper Vintage tournaments have seen steady gains over the past couple of years, and will likely continue to do so in the short term. That is not a sign of a shrinking format. The reason people are bailing on Vintage on MTGO is because it is a terrible program, with limited rewarding experiences, and does not even allow people to play all of the cards available in the format. I understand how it is a great choice for some people who have limited or no exposure to Magic otherwise, but the failings of MTGO are well documented now and have driven away many, many Vintage players who have tried it (and many of those same players who still play paper Vintage).
Sidenote Edit: If any of you see this post and have constructive input on how you would redesign MTGO (everything from UI/UX to tournament structure to fee/ownership structure), please email me at jaco @ eternalcentral.com with input. I am going to write a couple of long form articles on the subject, and do no want to derail this thread.