The limits for general purpose CPUs for the about a decade has been power/heat, not transistor size. In the 1990s-2000s, performance could be increased with faster clockrates and more on-chip caches. Since about 2005, when clockrates passed 3GHz, the CPU vendors embraced multiple cores and have cut power demands.
Moore's Law can continue with 3D chips. Maybe a CPU of 2025 will be built with a first layer of transistors that covers the entire areal plane just for caching and with additional layers built vertically for other uses. If so, the number of transistors per chip might be much higher than a CPU of 2016.