At the time Misty left for Johto, the only example we had prior was Brock coming back after he left for Tracey. When Misty originally left when May was introduced, Misty had been there in every single episode for 5 years at that point.

She also still got frequent specials/cameos in the early AG era, so the writers kept her character alive on the side even if she wasn't in the main series at the time.

But what did you think back then? Did you think Misty would eventually return, or did you expect her to stay gone?