They should have let Kes mature, have a child, grow old and die during the series.
Why show a gun if you don't plan on using it?
Why have a short lived species if you don't show its consequences on the crew?
They should have prolonged her lifespan and scapped that silly "nine-year lifespan" rubbish. It was totally unrealistic, didn't lead anywhere and had no impact on the series itself.
Not to mention that it was more than enough to have Naomi on the ship.
When I think of Voyager, it always seems like Kes' departure and Seven's arrival tends to split the series into two different series; and while there are more Seven episodes, I actually prefer the Kes era, since I think there was a freshness too the series (plus out of the two-parters and TV-movies I find "Future's End" (#1) and "Basics" (#2) too be the top 2 two-hour episodes) that left with Kes's departure and then we got all those old Janeway/Seven stories about their mother-daughter type relationship, plus many times I felt that the producers were just kicking back and just letting the show coast on he momentum that it had built up during the Kes era.
I agree. For me Voyager ends with "Worst Case Scenario". Then it became something completly different.