after they already had a better cable
Better how? There is nothing in the iPhone 15 that was improved after changing the port. For the iPhone 15 Pro, faster transfer speeds are now supported but if we are honest, how many iPhone owners will ever transfer anything to or from their phones using a cable? I remember doing so once in the past 5 years. Using the cloud or AirDrop is significantly more convenient. On a device as small as the phone fast charging at higher wattage than what Lightning already provided is most likely going to murder your battery.
Now there’s landfills of useless lightning cables, while we can still use micro-usb for a lot of other old devices.
I don't see why we couldn't still use Lightning cables for old devices the same way as we can use micro-usb cables for old devices? Apple and other manufacturers have sold billions of devices and accessories utilizing Lightning. It's not going to disappear any time soon.
