They replaced the only well developed character on the show with the subsequent only well developed character on the show.
I'd say the quality remained exactly the same.
How was Kes well developed?
Even the writing staff themselves admitted they messed up on her development.
Kes was as well developed as any other main character on the show.
As for the "writing" staff, well they are veeeeeeeery reliable, aren't they.
That's not saying much, considering that some of the characters on the show, Harry Kim and Chakotay in particular are some of the least developed characters in Star Trek history.
Again the problem with Kes is are few episodes about what makes her tick, rather then episodes where things happen to her and how she reacts to these things. There was less of character study with her then other Star Trek characters. This would have easy to remedy, but the writers seemed to have no interest in doing so, so Kes had the potential to be a great character, but the writers failed to follow through. She certainly a better character then Harry Kim or Chakotay.
I think this video has a good explanation on how Kes could have been used: