In my humble opinion, I disagree in some points. The size of the species kept is not the only factor that should make we feel in one way or another for cetacean captivity. If a bigger species is kept in an enough big tank and with the adequate care, there is no problem with that. In fact, an enough big and enough enriched tank is basically the same than a sea pen of the same size. Commeron's dolphin is small, but due to his needs of cold water it's much more difficult to keep in captivity than many bigger species, in fact not a small number of Commerson's died or had a short life because they was being kept in too warm locations like San Diego. By other side, orcas that are much bigger, are much easier to keep in captivity with good happiness and good reproductive success. Items like a too specialized diet, a need of deeper waters, and many other things are what makes a cetacean adequate for be kept in captivity or not, not (just) the size.
I would never call activists and zoos as two faces of the same coin. They're completely different coins. The activist tend to love individuals and ignore the species. At least for the radical ones like the members of PETA. For they, would be the same if all the endangered species of the world turn into asian elephants. I would not use the phrase "fight again those that ALSO support conservation issues". Zoos (or at least major zoos) support conservation issues. Activist do just the contrary. They are agaist conservation. And like Merintia said, I don't think that activist played a role in extinction of baiji and vaquita. They fight against zoos, but not agaist the species in the wild. The lack of a captive stock of these species probably were due to lack of enough worlwide interest in them before they become endangered.
About the solution, also in my opinion, this is not totally true. Information, explanation about zoo roles, etc would be very useful in change the mind of just normal people (most of which, sadly, dislike zoos or think in they as jails as if they were the old menageries of a century ago). But not activists. Most activists are a different level, and show them the roles of zoo conservation would be at most times like talking with a wall (a thread about this is somewhere in Zoochat, I think).