You can be as "original" as you like. There are plenty of people who are. There are not, however, plenty of people who read their books, because they read them and say "hey, this doesn't make sense."
The classic example is a mystery story. In your normal mystery, all of the clues are presented to the reader, or at least the detective is seen to acquire them, so that when the culprit is found, the reader says "I should have seen that all along!" Would you want to read a mystery where, after you've gone through a few hundred pages of the usual clue-searching, character-interviewing, and sneaking around, the detective who has been doing all this says "I was actually looking through the window when Mr. Boddy was killed, and the culprit was none of these people; the butler did it"? That would be a lousy book, because everything the author told you turned out to be meaningless in the last ten pages. You feel like you wasted your time -- because you did. The same is true of a fantasy story that doesn't make sense. If things can happen just because the author wants them that way, with no ramifications in the world (what
does happen when that water turns around and goes the other way?), then nothing you know, or think you know, has any bearing on the story ... and nothing the author tells you does either, because he might change it on the next page. There's nothing solid to hold onto, nothing you can understand, and nothing that makes any sense. It's like trying to follow the stream of consciousness of someone on drugs: it might make for a good trip, but not a good book.
Let's take my water flowing upstream example. You think that would be a good idea. Okay, in our new fantasy world, water flows upstream. What keeps it in the streams? Water that flows downstream stays in its streambeds because they're the lowest path it can follow. Dig a hole, and it pours into the hole; that's lower. To this upstream-flowing water, the "hole" is everywhere above it -- the sky, for instance. What keeps the water from flying straight up? It clearly isn't being confined by gravity, or it would be flowing downstream, so it will act like a whole stream full of Danny Dunn's anti-gravity paint.
Maybe you don't care. Maybe you've never thought about the fact that water flows downstream because gravity makes it do that, and for it to do anything else, the laws of gravity would have to change, or the attributes of water would have to change, and not-downstream equals
everywhere else. But most people do think of things like that. "Why doesn't it just fly everywhere?" is the obvious question. If you make water behave contrary to gravity, you've got steam in your stream.
So we'll call it "magic" and say that it acts just like water normally does, but it runs backwards on alternate Tuesdays, and because it's a BookCat novel, it doesn't have to make any sense. Okay, so our water is now running backwards today ... and that town that has required the local tannery to set up downstream now has a tannery spewing crud
upstream instead. Phew! And there's that farmer who wants to pour a bucket of water for his horse ... how is the water even going to get out of the bucket, since it's flowing uphill today? If the author ignores this, the readers are going to wonder what's going on; the author has to address this weirdness. And, like Chekov's shotgun on the wall, it has to mean something.
Exactly how you see the only options as being total disregard of reality or "cancer, or nuclear catastrophe" just boggles my mind. It's entirely possible to write a book that isn't "gritty" but still makes sense, and many, many people do it every year. I buy their books.
Incidentally, long ago in a D&D game, I
did have a small stream running backwards. It wasn't just window dressing. The characters, seeing this and knowing that it was nominally impossible, investigated how that stream happened to be doing something so uncharacteristic, met the student wizard responsible, and the whole next series of adventures stemmed from there. That wouldn't have worked in a world where facts are meaningless, as you advocate. Because water behaved like real-world water, except in this case, its actions meant something, and some very real people had a whole lot of fun finding out why (it was the student's term project) and how (a temporary time reversal), and then dealing with a whole mess connected to the school. Structure (including respect for facts) is a framework, not a straitjacket, and you can build things a lot bigger if you have a solid framework to hang them on. There is a reason there are no amoebas the size of elephants.
Would Dr. Who be any fun to watch if the Doctor could just pull out a big "I WIN" button and push it, and everything would be done for him? It would bore everyone to tears. It has, maybe not science exactly, but consistent principles behind things, and has done a moderately good job of sticking with them over the course of decades. Take that sonic screwdriver: it postulates force fields of some form (to do the actual unscrewing) controlled by a microprocessor system (to figure out what needs to be done). It can do pretty much anything you'd do with a screwdriver-like tool, and I suppose that would include a lockpick. But it can't shoot laser beams. It can't teleport its user to Mars. It can't turn into a whale. And viewers would complain if it did ... "so why didn't it shoot laser beams last week, when the Doctor needed something that did that?" And that's a TV show known for (at last for the last few Doctors) playing painfully fast and loose with reality. There is a reason, given the show's stated technology, why things in it
could work.
If you just make stuff up, you don't have a story; you have a daydream. I have plenty of those of my own, and don't need to buy anyone else's.