If Phineas would end up in a relationship with Isabella, the writers would have a choice: either make them go into a relationship and move on and forget about it or just focus the show on the relationship. They would be more safe with the first option because the show would last longer. If they chose the second option, the show would appeal to less audiences. Thus, ruining the show. You have probably seen this in the show Adventure Time where they reveal the secret.If they kept that same topic going for the rest of the season, the show would become less happy. *note* this is just my opinion on phinbella.