Apple has released a statement on their website about why they did it. I understand where they are coming from. They weren't up front, cos most people don't care about batteries and degradation and all the technical jargon. Most people just want their phone to work, so reducing performance behind the scenes was better, as opposed to letting it run at full CPU and their phone suddenly shutting down. Apple figured the latter wasn't going to give their users a good experience with their product.
I've read somewhere, someone used the GeekBench data to compare some Samsung phones, and they also reduce performance. Apple was chosen, cos they are the biggest with big bank acc. They are getting sues left/right/centre, cos lawyers need to eat too, and pay the mortgage on their McMansions and fancy cars.
And Apple doesn't really want their users to replace their batteries themselves with cheap ones that could catch fire. Apple has got good warranty, so when your iPhone fails within 1 or 2 years, they fix or replace with refurbished ones, like DJI. If you use knock-off batteries and your Mavic crashes, returning your Mavic on Care Refresh with the knock-off battery, and DJI won't honor it. But since you can swap out your Mavic batteries, DJI might not know. Apple doesn't want that risk, so they dont let users easily swap batteries on their iPhones.
As for DJI, would you like software to reduce the max speed of your Mavic when battery degradation is detected, or just let it drop out of they sky when you turn on sports mode?