Es correcto en positivo, puedes decir: "Hay quien cree/piense" "Hay gente que cree/piense" Pero para lo contrario (negativo) es: "No hay quien crea/piense" "No hay gente que crea/piense" O: "Nadie cree/piensa..." (Nobody believes) "No hay nadie que cree/piensa..." (There is no one that believes) Por regla gramatical "No hay quien cree/piensa" o "No hay gente que cree/piensa" son incorrectos.

Fíjate que en este articulo se usa "Hay quien cree..." y también "la gente que cree"... No hay gente que crea Hay gente que no cree https://www.elespanol.com/social/20190110/cree-viajeros-temporales-mundo-fascinante/367464402_0.html

Hi Spangola. “Hay quien cree/piensa...” is used informally. You can say it with out “gente” due to verbs creer/pensar, that are referred only to people, gives you the idea you are talking about they. This is called “omitted subject” in Spanish. But pay attention, if you omit this the subject, it’s better to say “quienes” instead of “quien” because you aren’t talking about one person but people. Does it helps you?