The thing about cartoony graphics, is that as graphics improve, they never hit the uncanny valley with things like the eyes or the mouth. In other words Sims 4 can go on for years and years in its simple cartoony graphics.
An example is that while you can clearly see an improvement in Pixar graphics over time, especially in textures and particle effects, they never look truly out of date. This is why the first Pixar movies were about toys, insects, and monsters and fish. Humans featured very little in them, usually as giant monsters, and children, who are simpler to animate.
At the same time, a Sims 5 would be interesting to see the next notch up. You know, if we ever see one. The main complication is, how would you approach realistic graphics with the current CAS system? It's always the eyes and mouth that are the most complex. If you look at Sims 3, you know it's the eyes that looked weird before.