While it may be true that Undertale is a resounding success, how much of that success can be attributed to Tumblr (the company) buying out 999,999 of the copies in order to advertise itself (as Undertale is clearly just a massive advertisement for Tumblr)? Nobody had even heard of Tumblr before Undertale launch.
There's also significant evidence that Anita Sarkeesian purchased over 5 billion copies of Undertale in order to continue to push Undertale's misandrist agenda, further skewing the numbers. In fact, I am going to make a YouTube video criticizing Anita for this right now.
Furthermore, some people are saying that Undertale's exploitation of memes, humor, and gameplay has drawn the attentions of Reddit, 4chan, and Anonymous--who knows how many sales THOSE websites have drawn in?! Those aren't real gamers.
Is it any surprise that a somewhat decent game, when released on a platform as dead and deceased as the PC, will be purchased by all PC users in an attempt to keep PC gaming alive? Of course not. One thing is for sure: I'm going to blame Undertale's success on Tumblr, Anita Sarkeesian, memes, Reddit, 4chan, people who disagree with me on NeoGAF, the instructor of that course I dropped out of, the government, and BioWare.