It is always great to have a campaign that spans for years. On the other hand as life so often gets in the way a campaign may sputter and fall flat well before you want it to. So you can spin up a new game in the same setting, or completely change games for a new feel.
Either way you probably don't want to run the same adventure that you just ran over again, even if the players have changed. But that means a lot more work on your part as the GM.
I've seen players keep a favorite PC from and transplant them from one game to the next, in fact that is a core concept behind the organized play of the RPGA and such.
But why should the players get all the fun?
I hold to the ideal that I don't need to make an extensive background for every NPC the players meet. Generally a few notes on how the act, and if the players interact with them more then I might flesh them out to give them a stats and so forth.
But sometimes I know an NPC is going to stick with the group either to shore up a party shortfall or as a story hook or something. But then I have to build a whole new character for the party each time... more work.
A fellow GM of mine introduced me to the idea of recycling main NPCs. He has a handful of NPCs where there is a personality that he has developed. Those same couple of characters who he has developed over the course of a game, rather than be thrown away, are lovingly kept and re-cycled into the next game he plays, re-imagined in the context of the new game, but still having the same general motives and personality. This way he already knows how the character is going to act and react to situations well before hand, because the character is already well defined in his head. To keep it easy he doesn't even change the NPCs name so it is super easy to keep track of.
But what if the players catch on?
The will, they do, and they have.
I was surprised at first, but then found that I related well to the NPC as the "feeling out" period was significantly reduced. Sure, it was a bit meta, but because we generally knew how the NPC would fit into the story the game moved smoothly along.
Of course you can always steal a beloved concept from one of your players have and make it your own as a reoccurring NPC.
I ran a game of shadowrun and one of the players ran a physical adept, with a lot of points dumped into temporary physical boosts (so yeah, he was a bezerker) troll with short term memory loss. He was smart (for a troll) so just above average mental facilities minus the amnesia, and of course plenty of physical ability and a nifty enchanted sword.
Stealing that idea an re-imagining it as an NPC I can have a bruiser type character who doesn't fall into the strong and stupid trope, and has the fun quirk of amnesia instead, so he's fine as long as he it with someone else but if left alone can completely forget the task at hand.