Since 2014, the silver team has had rivals, but only in China did some pundits like 1996 world champion Damon Hill actually sense the end of an era.
"It's like empires rising and falling and maybe Mercedes are on the wane," Hill said.
Mercedes, on the other hand, have been making fundamental strategy mistakes.
"In our sport it's inevitable that winning races eventually becomes more difficult - or you stop doing it for a time," Red Bull boss Horner said in China.
"Mercedes has a very fast car and it's amazing that they haven't won any of these races. But in general, our sport is fairly cyclical."
However, Mercedes boss Toto Wolff has said that the German team will not panic.
"I don't know what's going on," Lewis Hamilton said after China. "Since Melbourne we've gone backwards."
Wolff insisted: "We have to stay calm, put our heads together and solve our problems. Like everything else in Formula 1, it's complex."
The championship continues with the Azerbaijan Grand Prix in two weeks' time.