Google having a history of all the videos you watch via your account.
Even if Google provided an option to opt out of tracking there would be no reason to trust then since they have lied about not tracking people in the past.
YouTube seems to redirect any Premium profits intended to creators to the entity which made a copyright claim on a video. This would be sensible if YouTube’s copyright claim system wasn’t so vulnerable to abuse. Normal (yellow) demonetisation will pay out from Premium though. https://youtu.be/PRQVzPEyldc?si=5-wFn2SqPZLdOlqa
Features are removed from YouTube to incentivise Premium such as playing videos while your phone screen is locked.
Similar to above, Google have been increasing the amount of ads particularly on phones where ad blockers are harder to use. I.E. pushing users to Premium not by making the service better, but by making non-Premium worse.
Reasons not to buy premium: