Raising Pokémon simply so they'll evolve has never been Ash's style. He's never forced or even encouraged a Pokémon to evolve. People seem to have this unnecessary fettish with evolution thinking it will make Ash stronger. It won't. In any case a lot of Snivy and Oshawott's personality and appeal rested on them being an Oshawott and a Snivy. Oshawott would have lost his carefree and fun loving personality if he had become Dewott, the discipline Pokémon and Snivy would have lost her elegance by evolving to Servine. Evolution should only be utilised where it enhances or is necessary for the plot and it does not need to happen to every Pokémon. Like with Cameron's Riolu. It evolved in battle to win the battle and did so to demonstrate the strength of the bond it had with Cameron - Riolu evolves from friendship. To have Snivy or Oshawott randomly evolve for no good reason would have been akin to killing those characters off.
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A big part of Ash's most developed, loved and famous Pokemon are fully evolved like Charizard,Infernape and Sceptile. They don't have to say mediocre to be interesting. And if they aren't going to evolve at least make them competent and not a joke, this is why people dislike Totodile and Oshawott