Large rainbows are still very much a possibility if you are fishing the right streams. They run up from hatchery supported water (throughout the Park I might add) and can add quite a surprise to a fishing trip with otherwise small fish. The big difference is that these fish have to run up from water below whereas they were actually stocked in the Park many years ago.
Little River still has fairly good numbers of smallies from the Sinks down as well as over on the lower end of Middle Prong. More than once I have been excited at spotting nice fish on lower LR only to discover after getting a decent look at them that they are actually smallies and not the big trout I'm looking for.
As far as catching more fish many years ago, remember that fish experiencing more fishing pressure and or catch and release will become far more wary. As more and more people are fishing on the easy to access streams and releasing the vast majority of their fish, we must increasingly adapt to more accurately match the hatch if we hope to continue with reasonable levels of success.
Many years ago fisherman could get by with a handful of dry flies as well as a few wet flies developed specifically for fishing the southern Appalachians. Those days are now gone. With more and more fishermen focusing on match the hatch, the trout will be less and less gullible. Out west, fisherman will use what we would consider tailwater tactics even on the freestone streams. As more and more fisherman move towards matching the hatch, I believe that we will see more people fishing tiny nymphs and midges in the Park as we constantly try to find that new method that will outfish others.