Except for the fact that Kes wasn't a very marketable character (not that she was intended to be).
You have to admit that 7of9 checked all the Trek boxes, even beyond her cup-size. Super-intelligent, logical, biologically unemotional, naive in ways, stark contrast with the Captain character yet subordinate. She was the kind of character that should have been on the first season.
You're right about Kes not being intended to be a very marketable character.Personally I like her the way she was.
I don't think that Seven was that extraordinary and when it comes to the super-intelligent, logical thing, well we already have had Data. As for bringing her in from the start, no. What they should have done was to introduce her as the main villain in season 4, maybe as a Borg Queen. Then she could have been captured at the end of season 5 and in season 6 and 7 been transformed to a loyal crewmember (just as they did in season 4 but somewhat slower). Anyway, none of the original crew members should have been dumped.
When it comes to the Voyager crew, I think it was excellent as it was from the beginning. A good mixture of very different and interesting characters.