Pikachu is the primary marketing image of all incarnations of Pokémon right now. The chances of them writing him out of the animé are significantly less that the chances of them having Ash 'come out' as a cross-dressing girl.

That aside, I have noticed that Pikachu is beginning to have problems in facing other Trainers' Pokémon. Just look at his performance in Johto, for example. He only won two bouts, IIRC, verses a powerful Furret and against a Keckleon. This could be cured if Ash had the courage to train Pikachu in some non-standard attacks to cover its' weak spots (low Defence, despite being around lv.70+ now).

I am sure that Ash will only evolve Pikachu into a Raichu at the climax of the story, perhaps in the battle for the title of Pokémon Master. However, that doesn't stop Pikachu from learning a few moves like Reflect, Light Screen and Flash to boost its' combat survivability. Or even something exotic like Iron Tail or Hyper Beam to give him an unexpected edge against Pokémon that would normally have an advantage over him.

As Pikachu is actually a primary character (who often has a significant role in stories), I can't see them writing him out, I really can't. He is also massively more powerful than any new Pokémon Ash could catch. He is also Ash's personal friend, so he is unlikely to want to leave him behind. That aside, I wouldn't be surprised if one of the characters caught the positive/negative electric mouse twins. I suspect that they are deadly in 2-on-2 bouts.