Ok, for one thing Apple isn't slowing down older devices. Let's shut that theory down right here.
If you are referring to the battery thing, they were trying to help customers by making their battery last longer so they didn't have to replace it so the phone didn't die not even midway through the day or during an important call. Yes, they should have told people about the battery health, yada yada. But they were only throttling older phones with a bad battery. If you had that same model phone with a new battery or good quality left, you wouldn't notice anything at all!
The other reason for devices slowing down is that the more features you try to bring to a phone over time, the more the oldest phone in the supported list struggles with it. But people would whine if Apple dropped support. They are damned if they do, damned if they don't. But planned obsolescence or doing things to force people to upgrade is a bad myth!
People want the new OS for those new features! They want the new shiny thing. Can you imagine the feigned outrage if Apple timed WWDC with the phone release and made you buy a new phone to get lets say 95 percent of those features? Apple can do no right in some people's minds. They are an easy target being the biggest name in tech and the most valuable if not in the top 3 at any given time. It's just sad honestly. We should all be more grateful. They aren't perfect but they do care about consumers and today they've shown that in spades be it this news, the news about FaceTime on the web, privacy, or not dropping any iPhones on the supported list this year. But some people are just unpleaseable!