These men are hand-picked by both countries' selection committees - they have been deemed the best for their country. No player is useless. It is the nature of any sport for any player to have good days and bad days, for any particular player to have an outstanding day or several, for the top 'star' or two to have a really bad day once in a while. Who expected Stokes to be out for 1 yesterday?
Injuries can take their toll in a long Test. Speculation is that Archer may have been overused in Test 2, because he hasn't looked anywhere near the same since. Warner, for Australia, has looked pretty bad at the bat the entire series. But there's more to the game than just batting well and bowling well. Fielding is often overlooked. Weather and pitch conditions also play a huge part. At the end of Day 4, Paine for Australia wanted to put a seamer in there to bowl and the umpires wouldn't allow it due to bad light. Many, many factors affect performance, and that is what makes sports so great - the unpredictability.
Without Smith's double century in 1st innings of the 4th Test, Australia still put up 286 runs. That doesn't suggest uselessness on the part of anyone. They were down 8 when they declared, meaning that the nine other batsmen averaged nearly 32 runs each. For both sides, this has been some outstanding cricket. Australia has had the (slightly) better bowling.