I'm responding to this late so I'm sorry, but I was watching something recently that exposed a lot of music industry secrets.
The takeaway was - you can't compare current artists' record sales to historical ones. Ever notice how a lot of records are suddenly being broken since streaming was a thing?
Turns out the way a "sale" is recorded is so vague. Streams are counted, but a lot of fanbases are manipulating figures by creating playlists and letting them run on and artificially inflating them. Then you have things like YouTube ads that contain the song that are played for over 10 seconds being counted as a sale. Then you have YouTube views (a FREE service, don't forget) counting in with sales figures. You have streams which can be listened to for free, as opposed to having to pay for a physical version back in the day. You have sales being attached to merchandise purchases (in the small print of merchandise purchased from an artist it will say it includes the sale of an album and includes a QR code that can be used to download it. This code doesn't even have to be physical provided, it can just state that). There's a lot of shady dealings.
Sales could manipulated in the past (bulk buying purchases etc) but it depended on the record label's investment potential and was only done by the label. Nowadays, fans have multiple ways of artificially inflating sales too (I can't imagine the average person had the money to buy their favourite artists album 20 times back in the day, but they can certainly stream it on Spotify countless times, watch YouTube videos of it which count towards the data countless times etc).
Lastly, Adele's "3 million in one week" record was recognized by Nielsen, who started tracking this in 1991. They state that this the largest first weeks sales SINCE they started recording these figures. Which invalidates anything released before 1991. The albums of Michael's that could fairly stand against this were recorded before Nielsen tracked this data.
Long story short - I think Adele is as great as everyone else. But let's be careful about saying that she's outperforming MJs career in terms of sales. We're comparing apples and oranges here.