Are the "games as a service" games adding significant code after launch? I think most of the performance critical stuff is in the engine and core game loops. Content packs and seasons aren't likely to change those core bits, and instead be more assets and scripting intensive.
Destiny 2 just had to announce a complete re-do of how they're doing content now and in the future, because the game is significantly held down by technical debt from previous content.
Yes they do quite often. A certain amount of maintainance (bugfixing and optimisation) is required but also you are improving core systems to enable new capabilities and increase other developers productivity.