My question would be - how are you supposed to know when to replace the battery? You can judge your battery capacity fairly decently yourself - if the phone used to be at 40% after a day of hard use and now it barely makes it to bedtime you know its degraded. Measuring relative performance of the phone would be very difficult to gauge yourself. Granted, in both situations you could have other issues (unknown apps running etc), but if you aren't changing anything about your phone for the most part you'll know when the battery is not doing as good as it has. You then know to replace it.

Even if this isn't planned obsolescence, it is tangential to it. They know they are putting a new phone out every year or so, and they are hedging their bets that people will move to it so this decision makes total sense for them financially. It feels underhanded even if not fraudulent, they made a decision that is better for them than you. God forbid you have to charge your phone during the day?