I don't think the originator has to be better that the standby or the replacements.
I saw Linda Mugleston, who was an understudy and not a standby, in both On The Town and Nine, but she was amazing in both shows. I saw both Linda and Donna Murphy as Ruth in Wonderful Town and enjoyed them equally. I saw Linda go on as Luisa Contini in Nine, and she was excellent.
From the clips I've seen online, the replacement cast of the revival of Nine was excellent.