Kate Flack, who’s currently working as a Systems Designer on The Sims 4, has mentioned on her profile how she did research and ideation of new features for The Sims 4 to support a rolling 3 year content/live service plan.
As this is a rolling plan, it’s very likely that the team has extended / will extend the original plan.
This means that for at least next 2 years, The Sims 4 Team will be releasing The Sims 4 Expansion Packs, Game Packs, Stuff Packs and Updates.
Please note that this plan has probably been extended!
Brainstorming, research and ideation of new features to support a rolling 3 year content/live service plan.