I think it's pretty telling that Pagos completion was 35% but the players eligible to enter BA is only 19%. That's clearly a drop off in interest in the content. I'm in that 19% but I've never set foot back in Eureka since completing the relevant quest. There was no retention in my case.
It's also weird that 47% of players stepped foot into Zadnor but only 35% made it to Rank 25, which was required to enter The Dalriada and see the end of the story. Again, it shows a drop in interest in the content though not as drastic as the one for Eureka. There's nothing listed that indicates how much of that 35% continued to do content in Zadnor after reaching Rank 35. Completion isn't the same as retention. I feel like the percentage of players that completed the Critical Role IV achievement (do 500 CEs in Bozja/Zadnor) would be a better gauge of actual retention.
I'm not saying that there shouldn't be any more exploration/field operation zones, only that what you're listing doesn't support any claims for retention. More content is a good thing as long as a significant portion of players are doing the content so the development resources and time aren't going to waste.
But the dev team is still hindered by resource limits. SB had both Deep Dungeon and Exploration. ShB only had Field Operations (curious drop in content). This expansion only had Deep Dungeon but they also added Variant/Criterion as a new content form. Was it because Deep Dungeon had that much more ongoing participation than Eureka and Bozja/Zadnor did, or is it just a matter of they only have resources for one or the other and they decided it was Deep Dungeon's turn this time?
Plus there was no mention at the FanFest of either Deep Dungeon or exploration/field operations zones. Maybe they are adding one or both but saving that information for the EU FanFest. Maybe they are still undecided.
My suggestion would be for players to vote for what they want to see in the future by continuing to participate in it. The things we choose to do in game (or ignore) speaks louder to the developers than our words in the forums, reddit or social media.