I generally prefer shows go out on their own terms, before kind of gradually dipping into diminishing returns. It's happened before with too many shows I like for me to want to have Parks & Rec for another four or five seasons if it means the show doesn't mean as much to me anymore. Challenges for the characters are part of what make shows interesting and fulfilling to me, and if the characters get too complacent they run in danger of losing the essence of why I came to care about them in the first place.
That said, some shows handle long lifespans really well and there's certainly the talent involved with Parks & Rec to potentially manage it. I know Mike Schur and Amy Poehler are both super influenced by Cheers, which ran for 11 seasons and handled it as best as might be possible.
And I certainly wouldn't begrudge anyone for whom challenges and meaningful plot progression don't matter as much, who just want to laugh and see characters they like. That's fair, just not what I look for.
My general rule of thumb is that dramas should tell a focused, concise story and comedies should go on for as long as they are able to make me laugh a couple times an episode. If Breaking Bad was stretched out for eight seasons I'd (probably) be pissed off (or at least mildly annoyed). On the other hand, I don't mind the overstay of a show like How I Met Your Mother, which has clearly been in decline for a long time.
There's so much bad TV out there, I have no problem with a show that I enjoy delivering laughs on a consistent basis...especially one as warm-hearted as Parks and Rec. It's perspective and general niceness towards all its characters is really unique among shows that I watch.