I fail to see why some people are so insistent on having a Wimpod that doesn't evolve on Ash's team, I mean seriously there better options then Wimpod if you want a pokemon that can evolved but doesn't on Ash's team, and before some one says oh well Wimpod has plenty of moves that it could use from it's move pool for it's move set, the problem is if you look at Wimpod it only learns two moves through being raised and that Struggle Bug and Sand Attack all the others are Egg moves or have to be taught by TM, and sure while their has been pokemon on the main cast before that had egg moves or TMs moves in their move set the problem is with Wimpod you would essentially have to have either it's entire move set composed of those moves or have 3 of it's moves in it's move set composed of those move, the problem here is evolution for Wimpod is to beneficial for it to pass up.
There really isn't any argument you can use that would justify Wimpod not evolving and if you worried about Guzma having a Golisopod you could either make it shiny or better yet Ash could just not get a Wimpod to begin with. I mean in many ways it's like saying oh well if a Magikarp wound up on the main cast it could just stay unevolved, look like Magikarp there is no reasonable scenario where you can honestly expect Wimpod or Magikarp for that matter to beat their evolutions in a way that wouldn't seem like complete BS, and before you try to bring up stuff like oh well Pikachu has beaten Raichu before and Buizel beat it's evolution so it's been shown that pokemon can beat their evolutions, the problem with that argument is that all the pokemon that have beaten their evolutions were in as bad of a position as Wimpod, as they had some power to back it up where as Wimpod on the other hand doesn't and it were to attack a Golisopod it would probably feel more like a light tap to Golisopod.