Live service games come with inherent constraints that limit the sort of things we can do. In exchange, they provide stability and sustainability because players who like the game will likely continue to like the game as we make more of it for them. As such, the differences between live service vs new releases tend to be opposite extremes on the same spectrum. Here are some examples.

Scope
New releases are expected to have broad scope and to bring new things to the table. Players want new and different things from new games (but not too new or too different). Not making a sufficient differentiation between a predecessor or inspiration will hurt a new game's reception.
Live service games cannot swing for the fences, because the entire game is already up and running. Everything that has come before must continue to be supported forever because players still have that content and expect it to work. This limits the kind of changes we can build - too much has come before it and sets the foundation, which makes massive changes risky and dangerous. Look up New Game Enhancements to Star Wars Galaxies for an historic example. We must build stuff that works within the existing game and can play nicely with the existing content. Further, live service games must have periodic content drops, which means we cannot work for years before we release something. In order to keep players playing and interested, they need regular content updates to keep interested.

Sustainability
Established live service games are much more sustainable than new releases. We always hope that a new game will find success, but it's often very tough to get players to commit hard-earned money to a new game when they already have established favorites. Live service games already have the hard part done - they have an established audience that likes the product and continues to play it. Providing them more of what they clearly want is a lot easier than trying find a sufficient audience to sustain an entire new game.

Complexity Creep
Related to scope, there are many constraints in place for live service games. We cannot replace existing systems because the game is already live. We cannot take things away, because players have already paid for that content. In rare cases we will go back and modify legacy content (e.g. World of Warcraft doing a stat squish), but this is not the norm. As such, live service games tend to grow in complexity over time. We continue to add new systems, new gameplay, new features, but we cannot take things away to maintain the same level of complexity. The older a game gets, the more complexity it necessarily accumulates from all the additional content that we add. This eventually hits an event horizon where new players find it incredibly difficult to advance due to the complexity of the elder game and the game shifts to a retention strategy of existing players.

Crunch
Because scope is necessarily constrained and systems and tools are already established, there's usually reduced crunch when working on a live service game. New releases are much more ambitious, trying new and untested gameplay which leads to unforeseen issues and necessary iteration time. This often causes the workload to drastically increase over time, necessitating significant developer overtime.
These are just some of the common aspects of working on a live service game (or live service adjacent games, like annualized titles) when compared to working on a new release. There are clear tradeoffs from many perspectives - creative, technical, production, physical, and so on. New releases can be super ambitious, but that ambition comes at a hefty human cost. Some are willing to pay that cost and some are, unfortunately, coerced into doing so.
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