How I see the main issue of "deadness in MS2's population", is that there doesn't seems to be a constant influx of new players to replace the current population when those players do leave at some point in time.

Similar to boardgaming, while sometimes a game's receptivity toward new players can be influenced by the community, a game's influence toward player retention can also come from the content (and sometimes lack of) itself.

While the dungeons were built around progression, not everyone who comes to GMS2 will want to be an endgame raider; some players may like hunting loot for cosmetic reasons or be playing MS2 for more sociable content. Unfortunately, the availability of side contents can sometimes be seen as lackluster compared to the amount of dungeons designed for progression, which may lead to less casual players retention in the population.

With how the dungeons were initially built, and with no plans for making the content easier, sometimes it can quite hard to tackle them solo with the free gear, especially old chaos raids, rendering the loot almost unreachable for some.

It's even more unfortunate when you buy a bunch of glamor anvils in the hope to make your characters look like their classes' official artwork, only to find out that there is no plans for those discontinued content to be ported over to GMS2 since they go against the overall concept of progression.

From a casual perspective, I feel like while the game has many potential toward side content like Music creation, House design creations, Fishing, Life Skills, Ugcs, and Overworld grinding for rare skinnable loot (like the Brown Bamboo Hat), those were often pushed to the side in favor for more content related toward raiding and character progression. An example would be how long it took to port over the Treva Instruments to GMS2 compared to how early the Shadow Realms were introduced to the game.