I have to agree that Serena Williams era is over as new young players are starting to come through and be competitive against her. She could still win a Grand Slam here and there but Serena's dominance is certainly over now. The same with Nadal and Federer, there dominance has been over for a couple of years now and now it's Murray and Djokovic.
Murray's dominance didnt work against Kei Nishikori, he is mentally drained with all the matches he played in the past few months without any major rest
You cant count Serena Williams out,she is just a dominant player and strong one if she is injury free.Age is not a hurdle for serena. Have to see some dominant consistent performers from somebody else to boldly say that the Serena era is over.