If I know Ash/Satoshi he might've had the intention of leaving Pikachu but you know he'd probably end up going back for Pikachu even if it didn't show up at that moment. You saw how he ran through the forest and then stopped. I don't think he could bring himself to leave that forest or ever leave Pikachu for good. He might be able to make it without Misty and Brock as we've seen but can he make it without Pikachu? I think not.