Now back to your thought on why they didn't just add to the time machine. I think that is also backed by how now you can see your descendants in the future and even how what you do in the present creates changes in the future in the new expansion. I believe that this expansion is a kind of view into what the developers see as the sims future.

The new EP is based off a new world, the Sim future, where simbots have evolved into Plumbots and where simple time machines have been made into time portals. I have an idea as to why the simbots and time machines were not just revamped like the Vampires were.