SimGuruLyndsay confirms The Sims 4 has no planned End-Date.

Must confess to having reservations. The underlying game engine does not come across as being robust enough to handle infinite packs, the graphics quality will eventually suffer and become dated, and the already masssive bug problem will only worsen the more stuff they add. The more complex something becomes, the more likely it will break, and the harder it will be to find and fix the problem(s). Too many moving parts.