APIs and new features are the main reason why phones slow down. This is not a theory. For companies like Apple, it's all about priority. Even with huge resources, they are limited - this is why we get feature releases and polish releases of iOS, macOS, Windows, Android, etc. Usually it's a mix of two, but you can always notice what was a priority.
Apple does not intentionally slow down phones, however, we live in a world where people have higher and higher expectations and want to be amazed every year - so Apple, Google, Microsoft - they are all in a constant race to provide new, exciting features - and in this race, Apple put the emphasis on new devices and didn't put enough effort into older devices. It's not intentional obsolescence - it's not caring about obsolescence. And with the recent events, they were pushed by their users to care more.
Of course, there is a downside to everything - just look at all the "iOS 12 is more like iOS 11.5" kind of comments on this thread alone. Some people are let down by the lack of new and exciting - and it just proves the exact opposite of what you wrote in the post: yes, it's more than possible to support older devices, if you take the time to optimize and polish everything at the expense of exciting new features. Or, they could've made all the "I want new toys" crowd happy but made something that would probably make slow devices even slower. If you follow rumors, you know that the idea was to have a dramatic home screen redesign for iOS 12 which was pushed for next year - probably due to all the "slowing down" complaints.
So, again, what you call a 'theory' - is just how things are. In a perfect world we'd have optimizations and speed improvements alongside new features - but that's just not how it works. At that scale of deployment, even Apple has (relatively speaking) limited resources - it's either one or the other. I am glad they chose to make older devices run faster and optimize iOS 12 - if they succeed in this by the time it's out - I say it's worth it. And, no, it doesn't make the claim that new features are slowing down older phones less believable, if anything, it only reinforces it.
People - iOS 12 is a good move by Apple and a result of people saying they want them to put more effort into older devices. Which is something Apple brand should be (and usually is) known for - making things that last. Let's not be negative about everything - because this is exactly what we asked for.