I agree that it's entirely possible they brought in 7 of 9 as a sex symbol initially (one which grew into something more).
However, I have to wonder, just how much of the decisions were made as a direct result of network interference.
Don't get me wrong, I still think Voyager beats DS9 and TNG, but Berman and Braga apparently wanted to serialize the show at some point to flesh it out more (only to be prevented by the network).
If Voyager got something along the lines of 4 episode stories like Enterprise did, it could have showed certain situations in a bit better capacity (Scorpion for example or the Equinox).
Maybe even preserving the Nova class ship to tag along - plenty of options to consider.