1. He won't catch all Pokemon. At this rate he will barely exceed 100, so at most he might end up with 151. He's only caught 16 Pokemon so far this year.
2. He initially didn't even want to catch Scorbunny due to his obsession with Mew, but then to rationalize his desire to have it as a partner, he claimed that catching all Pokemon would bring him closer to Mew. The logic might be questionable, but the point is that he enjoys catching regardless of Mew and you'd have to be very oblivious not to notice that.
3. Project Mew was hardly part of Go's plans. He initially rejected it because he wanted to find Mew by himself. Changing his mind is called character development, and Gary had a lot to do with that.
4. Just saying, but we still have little idea of how winning the PWC fits into Ash's Pokemon Master goal exactly. A champion is meant to defend their title for as long as possible, but it's obvious that Ash doesn't care about that. Imagine Leon losing to Ash only to realize that the latter doesn't even care about the title and was just proving a point about being superior to him. It happened with Drake and Brandon, after all.

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