Somewhere around the turn of the 20th century a famous scientist of the period gave a lecture in which he announced the end of physics. Perhaps someone here recalls who this was, I couldn't seem to provide the right keywords to Google to track him down, but he said that physics had already discovered the important principles, and that scientists in the future would just be adding decimal places or precision to that that was already known. Sure, there were still a couple minor problems out there, like the spectrum of black body radiation and the inability to detect the ether, but those would no doubt be wrapped up soon.
It wasn't suspected at the time that the world of physics was on the brink of a revolution that would introduce quantum theory (explained black body radiation) and relativity (explained the absence of the ether). This tells us that we can't know where the future of physics will take us until we get there.
There is one way in which we could be realistically said to be approaching the end of physics. The low hanging fruit of physics, the simpler and more obvious principles like mechanics and optics, were discovered long ago, and the 20th century showed that further progress in physics required increasingly greater collaboration and time and money, often on massive scales, and this trend continues. So while past experience tells us that there is still far more to learn, practical constraints might make further significant progress too difficult for mere humans. In other words, if we are actually encountering any limits, they're not limits of knowledge but of human capability.
--Percy