Whereas if one doesn’t have prime, there’s many other retailers.
According to this article, if you use Prime, you cost Amazon money.
If you subscribe to Prime in any other way, Amazon might make less profit, but you're not costing them money, you're 'feeding the beast.'
A sale on item “x” is not a loss if it’s net effect is to bring in massive profits. If there was no overall monetary benefit to prime it would be peculiar for Amazon to keep it.
The customer pays for the prime shipping.
Now, rant about Amazon Music incoming!
What I don't like though is all the spam Amazon does in Amazon Music to try to get you to upgrade to Amazon Music Unlimited - honestly, it's absolutely relentless! Multiple times a day it brings up a full-page and/or popup nag, and sometimes even does it in the middle of a track! I've been a member for several years, and must have said "no" thousands of times by now - you'd think the system would have realise by now that all they're doing is damaging good will.
You said you haven’t canceled your prime subscription, but have you started using the service less? I think this is a case where the relentless A/B testing ubiquitous in modern tech fails: how would one even measure this long term sentiment that may not affect user behavior for a very long time?
Although, as a user I’ve learned to complain by canceling (and explaining why when possible).
You're right that it's difficult to measure goodwill over long timescales, but IMO not everything needs to be 100% science driven - the team behind Amazon Music must know that excessively, relentlessly nagging already paying customers is going to a not them.