A couple thoughts here.
1. Asmongold and Rich, specifically, coming to the game and the influence they, and other streamers had on the population, was an anomaly. The game's had consistent growth trends from the start, but that acute spike is without a doubt an outlier. How many of the folks they brought to the game will stay for the long term? I've got no idea, but I'm guessing not a huge percent as they came from WoW and XIV's so not like WoW that I can't see it retaining a lot of those folks. Had Blizz not screwed up so badly in multiple ways in a short amount of time, I doubt we'd have seen the exodus we did.
2. "The current population is now lower than during that Pre-EW content drought...". It's normal in this game for people to sub for the start of an expansion then come back a few months before the end and get caught up on things before the new expansion hits. I've seen this in my FC and friends list regularly. We get a decent bump in active players 4 to 6 months before an expansion so we get an inflated pre-expansion population relative to the X.0-X.1 content drought. We saw this effect again, except multiplied by many times, when the streamers came in. For those of us who keep playing, it was a content drought, yes. For those returning after a hiatus or for all those new players, there was a lot of new content available/no drought.
3. EW may indeed have a lower retention rate among long-term subscribers; without more detailed data it's hard to tell. What I believe is more likely is that the Bandwagoners, for lack of a better term, came, played the game, causing a massive but short-lived population spike, then left for other things. This is less about retaining players on an expansion-to-expansion basis as it is retaining players who only came here to have something to do when leaving WoW and following their streamer of choice. Making any sort of comparison between that anomalous spike and retention 6-12 months later isn't going to work well.
4. What would be really telling would be to see year-by-year retention numbers of people who started the game pre-WoW Exodus/pre-COVID and separate retention numbers for people who started playing during the WoW Exodus/COVID. I'd guess that the pre-WoW numbers would track with long term trends and we'd see a significant drop in the Bandwagoners when Elden Ring, New World, and Lost Ark launched.
I would LOVE to have SE's raw demographic information and a really dense survey of the population to dig into. That would be incredibly interesting to tease apart.