The reason Moore's law worked so well is largely because of this. When we began creating transistors on integrated circuits, the only thing preventing us from making them exactly as small as they are today were things like cost and the capabilities of fabs to make them reliable and stable. So ever since we started making them we've stepped down the size by about half, rather than something smaller that would be more difficult and expensive to make. The entire manufacturing industry gradually became more efficient, increasing reliability and reducing the cost of fabricating the integrated circuits.

The history of performance speedup hasn't been solely due to doubling the transistors. Other types of architectural improvements in memory, buses, parallelism, multiple cores/threads have yielded performance doubling or more. Memory access has always been a huge latency and there are lots of ways to improve that without just making the transistors smaller.

We have a lot of work to do still! The majority of our computing has been on the x86 architecture, which basically stems from the 8080 which was designed in the mid 70s. We can still discover new and better architectures and materials. It's just the standard shrinkage of IC's that was the basis of Moore's Law originally has reached its limits.

That's also ending, as transistors will not get cheaper and chips will probably not get larger in the future.