Benedict Evans runs some numbers and estimates there are 6.8 million Nexus 7s and 680k Nexus 10s in use, give or take almost 50 percent. (You estimate with the numbers you have, realizing it could be off.)
Anecdotally, my Nexus 7 has gone from “able, if uninspiring, small tablet” to “ugh, you piece of crap, I hate you” status. As an e-reader and light gaming platform for my wife before she got an iPad mini it was fine. I then tried to turn it into a media device for my office desk and found that at just 10 months old the battery doesn’t hold much of a charge anymore and the audio bleeds interference from either the cordless phone or microcell tower on my desk. Not so with the old iPhone 3GS I’m using instead.
It shouldn’t be that surprising that a $200 device isn’t built all that well. What should be surprising is analysts and pundits who automatically assume a low price means a winner.