The days where AAA games dumped the engine between releases are long gone (like Playstation 1 long gone). For franchise games there will be incremental improve

The days where AAA games dumped the engine between releases are long gone (like Playstation 1 long gone). For franchise games there will be incremental improvements to engine systems; some to support new gameplay features, some to help with development tooling/pain-points, and some to improve visual fidelity. The trick is choosing the correct number of upgrades/refactors that fit the release schedule while remaining relevant and not dooming your next franchise release with insurmountable feature debt. In my experience the biggest engine changes come with new hardware platform support, typically a console generation jump. More power requires a toolchain that can handle more complex assets (and more of them), new graphical features and expectations, different optimizations, new API, different storage types or capabilities etc. Bigger franchises may use a new console generation as an opportunity to gut an existing engine and rework to the strengths of the new hardware generation (and remove the support for older hardware, such as removing 32bit pointer support, DirectX9, Windows7 etc). Historically where game engines get in trouble is when a codebase is written to one franchise and then forced on to a team making a fundamentally different game with fundamentally different requirements (open world vs compact maps, physics heavy vs platformer, single player vs MMO). At the executive level it makes total sense, they spent x million$ on a game engine and are told by everyone how great an investment it was... why not spin up a new game using that amazing (and paid for) technology? The flip-side has also sunk numerous game engine efforts, which are often buried without ever becoming public. Doing everything all at once is impossible, you can either do many things generically but with restricted performance and capabilities. Or you can do 'everything' required by a specific game and slowly introduce features that support other games and make things more generic.