True that it's pretty much impossible to prove a motive with 100% certainty, but some facts make a strong case...
1) Why throttle devices based on battery health if it's still under warranty and considered "Healthy", and Apple won't replace it (even when asked to)? It can't be healthy and need throttling. It's even throttled while plugged in.
2) Why did Apple remove access to battery health information right as they started throttling based on battery? That one seems awfully deliberate.
3) Why would Apple not recommend a battery replacement to those who come in with a slow device? Reports from people suggest that Apple Geniuses explicitly told them there was no relation between battery and CPU, which is now known to be untrue.
4) Why was all this kept a secret? Throttling down to 40% of CPU performance is an enormous hit in performance. Slow/sluggish performance is one of, if not the most compelling reasons prompting people to upgrade --- which Apple can't pretend they don't know as performance is among their most advertised features of new phones --- so why wouldn't they let customers know when it's happening, rather than deliberately hiding it?
Their actions led to consumers making ill-informed decisions, to Apple's benefit.
We should add to this. I will keep editing as I come up with stuff.
5) iPhone 7 being throttled just one year after its release, soon after Apple released new replacement models for it.
6) Apple only admitted to throttling when faced with overwhelming facts. Without such facts and Apple's admission, customers would be left even with more throttled phones with each passing year/iOS update iteration.
7) New iPhone sales have been lagging since 2016, except for when new models come out. And majority of activations of iPhones for 2017 Christmas was still the three year old iPhone 6. "The iPhone 6 and 6 Plus took top spot with a combined total of 27.5% of activations followed by the iPhone 7 and 7 Plus with a total of 25.5% of activations" Indicating there was still demand for older models and reluctance of people to switch to new, more costly models, which would affect Apple negatively.
8) Apple defends it's decision to throttle as helping customers prolong the life of their phone. But this is inaccurate because all throttling does is reduce the wear on phone's battery to prolong life of a supposedly degraded battery. The phone itself is perfectly fine.
9) Customers with throttled phones are never aware of failing battery, nor of their phones being throttled because of it, and reduced performance is never a symptom of failing battery. So, a degraded battery is never suspected.
10) Phones do not degrade, except for battery which is replaceable. But Apple introduces software updates with new models in mind, new models which are capable of handling higher demand and have higher CPU. However, a customer with an older phone is unable to make a choice between security/bug fixes and non-essential enhancements features. Therefore, newer and newer software themselves, because of demanding non-essential features introduced and forced, slow down older phones as they require more time to make computations. There is never a choice given between the two for a customer who just want security and bug fixes because of increased hacks/bugs and one's increasing usage of phone as their wallet.