It kinda was. In the First episode, Ash saw Ho-oh. Where is Ho-oh? In the SECOND Generation. This means that they actually had a Second set of pokemon already ready to go. Another point is that throughout Kanto, Orange Islands, and Johto Ash a Misty's relation ship skyrocketed O.o Idc what kind of 'ship' you support, this is a fact. Hell, in The Movie 2000, she was even teased as his Girl Friend -_-
BUT once the show got REALLY popular, they decided to scrap progression, stay with their Target Audience (Kids) and just 'Reboot' the series every Season with new pokemon. They even had to get rid of Misty because Ash and her relationship was becoming 'too much'... That's the only reason they switch the girl every Season, they need to keep it a Kids show.