The cool thing about mk is that it was initially a licensed game starring Jean-Claude Van Damme. He didn't want his likeness to be out there in the long term, so it fell through. Johnny Cage was born under these circumstances. The team noticed that the titles of other fighting games were a tad cheesy, like final fist and whatnot. Then someone ad written the word "Combat". Then someone changed the C to a K, to make themselves unique. Mortal was added after that, and the rest is history.
MK putting guest fighters in isn't short of what they wanted to do in the first place, tying something that has been known to the world into a game. I've thought this through and I want to share. To answer OP's question, that "we" is a minority in the world of MK. I MYSELF do not care for guest characters, because MK's atmosphere will naturally bring in a darker guest to begin with. I don't care for Jason, Predator, Alien, Leatherface, Pinhead, Pennywise, Freddy Krueger, Michael Myers, Spawn, or Ash Williams. I've seen a handful of content from some of these franchises, and admit that Freddy V. Jason was a cool movie to watch. I do not like horror movies in general. I noticed something when I saw that I had all characters in MKX, completeness. I was satisfied with seeing the character select screen full, and nothing grayed out. Alas, I do not care for playing the four guest characters.
With this new MK coming out in just over four months, I have the opportunity to plan ahead with what I want. Unless a future guest character interests me, I will not buy one. Now I understand nuance. Nuance in this case is towards people liking guest characters. People will have different tastes from me. I will acknowledge that. I do not want my opinion to stymie the flow of what a company wants to bring in. Someone's favorite character in MKX could be Jason or Predator.
Guest characters can bring in lifelong MK enthusiasts, all because Freddy Krueger was announced. It can allow people to share their favorite game with a friend who previously had a passing interest, if no interest at all. It can spark something. Now I'm not saying that we need guest characters to have the franchise to survive. But the enthusiast is a small minority compared to people with expendable income and want a coll looking game.
There are regular, MK based characters that people do not want in the next game. People can see a leak and get angry that "BRC got in before Tanya". I understand that, but I would not mind if Tanya got left out for BRC to get in. That is my OPINION. I will not partake in guests unless they interest me. Like others have said, vote with your wallet. I will.