Because most people didn't realise that an older battery would cause their phone to slow down with zero notification. They just see that they have an old device

Because most people didn't realise that an older battery would cause their phone to slow down with zero notification. They just see that they have an old device that's slow, and assume it's too old to perform well with modern software. The experience of old hardware feeling slow after a bunch of use and software updates is common. The experience of hardware slowing down with no notification when the battery is getting old is not. I'd actively prefer random shutdowns over non-notified slowdowns-at least the former indicates that tech support is required.