Your argument is like saying McDonald's is better than what Ferran Adrià can cook because they have sold more, or that Gasolina by Daddy Yankee is better than Dvorak's "From the New World" for the same reason.
Something created to appease mass market can be extremely popular either because of its entertainment value or how cheap it is, but it will objectively be of inferior quality, not as a product, but as a work of art. There's nothing wrong in enjoying a cheap of entertaining product just because you like it: I like plenty of "pop" stuff myself, but there as mentioned there does exist objective criteria to recognize something as an actual masterpiece.
I'm not sure why would die on such a hill. I'm guessing you studied something more scientific and have never had any particular respect or interest in literature or writing? I can understand that, but do keep in mind that you're perfectly entitled to not liking Shakespeare or Orwell, that doesn't make your taste good or bad, nor some of their work any less of a masterpiece.
I didn't enjoy reading Don Quixote de la Mancha, so I wouldn't buy it for myself or read it again. Mostly because it's too full of clever references to its own day and age that completely fly over the heads of current day readers (actually quite similar to Alice in Wonderland, but the more fantastical setting keeps that one fresh regardless so I did quite like that one), but I acknowledge its worth and the fact that it's a masterpiece for it's literary merits.
TL;DR: masterpieces exist and sales isn't a criteria. Anything like 50 Shades of Gray can become a best seller, but there is nothing to praise whatsoever about the way it's written.