It's a common issue at this point: a game releases, gets years' worth of updates and DLCs, and then eventually the developers move on to developing a sequel. The sequel comes out and... the depth and amount of content is nowhere close to what players have just been experiencing in its predecessor. The sequel may have many of the quality-of-life features that didn't arrive in the predecessor until later updates, but it simply can't launch with a full game's worth of content plus years of DLC's worth of content. It only gets worse for games that support modded content, too, because they'll have years' worth of mods on top of the developer-created content.
We've seen this a lot already: the Civilization series is infamous for the sequels not living up to their predecessors until they've had years of support themselves; Crusader Kings 3 was seen as lacking in long-term replayability for passionate fans of the series; Destiny 2, upon release, was seen as shallow and sparse compared to the first game; and, recently, Cities: Skylines 2 developers spent the lead-up to the game's release trying to reel in expectations because they didn't want fans to expect the game to have comparable amounts of content to everything that's available for the first game after eight years of post-release updates and DLC.
To compound this, many of the games that benefit from extensive post-release support are less story-focused games. They often offer a mechanical foundation and a sandbox wherein players can create their own experiences, stories and lore - Civilization has no plot, nor does Cities: Skylines or Crusader Kings. They're similar, in fact, to tabletop RPGs - like Dungeons & Dragons - in that sense. And they share another issue with tabletop RPGs: sequels sometimes just aren't necessary. When there's a new story to tell in an existing world, or for an existing character, it obviously makes sense to make a sequel and tell that story. But if the game is more of a mechanical foundation that's already sound? Well, major overhauls to that foundation are a reason to make a sequel, but sometimes it can just feel like "reinventing the wheel" for the sake of releasing a sequel, not because it's necessary or because it improves anything.
It feels to me like a problem that will only become more and more pronounced as more games opt for live-service models or extended post-release support, too. Can anyone think of any examples of games that had extensive post-release support through updates and DLCs where a sequel was then released that wasn't seen as disappointing or a step backwards?