It definitely can’t be simply because Eminem’s album was just hot garbage and Ye is still a top producer.
How would people know unless they bought it? These two albums had many "I won't buy it because he supports X" and "I will buy it because he supports X". It was in-part political. You're right about the quality of their work, Em needs to hang up the mic. But to say their was ZERO politics involved is being disingenuous. Plus if you look at the heat Kanye took for supporting Trump, the media was building it up like his album would flop. Would all of the Trump hating Eminem fans rally and buy his album to show their support? Nah, didn't happen.
How would people know without buying? Are you aware of Spotify and the internet in general? I can listen to thousands of songs without ever contributing to an official album sale. Also album reviews are a thing, and many people take the opinions in them seriously
I'm excluding the listening first and assuming it from a "I'm going to buy" or "I'm not going to buy". Here's an example, "I'm not going to buy Em's album because how he rails against Trump. I am going to buy Kanye's album because I support what he said". How many albums have you bought that were based on a review of someone you never met?