Question

What will happen after you reached 100,000 Watts and unlocked Quiet Cave?

Answer 1

Well, I bet you noticed that every route on the pokewalker, has different types of pokemon. Most pokemon you see are some you cant catch in the games. So nothing happens except the chance to catch more rare and cool pokemon. Also I beleve that it wont restart or eny thing, it will just be the same exact pokewalker ever since you got Heartgold or Soulsilver.

I hope this answerd your question.

Comments

Yea, but what with Watts? Are they still added to game?

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You probably still get the watts but they wont do anything.

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After you unlock Quiet Cave, yes, your game does in fact still collect Watts; though they will do nothing in the game except show your total and steps.
You should always try and spend as much of your Watts as you can before you return your pokemon to your DS. (Of course after you've unlocked Quiet Cave)