This isn't even remotely true. It's possible to push boundaries and appeal to wide audiences; at no point have those things proved mutually exclusive. Horror movies have been appealing to wide, mainstream audiences for more than a hundred years. Universal Studios owes their entire existence to the popularity of the genre. And those movies might seem dated and old-fashioned now, but at the time? They pushed boundaries.
Here's the best way I can sum up my feelings on Terrifier (and yes, I've seen all three of them): When I was a kid in the mid-90s (like 8 or 9), I was at a sleepover with like three other boys my age and we watched the original NOES. First horror movie any of us had ever seen, and we lost our minds. After the movie, we started talking about all the other horror VHS boxes we'd seen at the rental store. None of us had actually watched those movies yet, but we were so caught up in the adrenaline from NOES that we got carried away describing them. Like someone would go "I heard in Candyman, the bad guy poisons little kids with candy." And then not to be outdone, someone else would go "I saw Hellraiser, and there's a part where the bad guy shoots nails out of his head and it kills a naked girl and there's all this blood everywhere." Each kid had to one-up the next kid, until we just straight up inventing movies that didn't even exist because we wanted to come up with the goriest thing we could think of.
That's Terrifier. There's nothing inherently original or creative about it. (And reminder, I've seen all three.) It's just a litmus test for gore hounds. And that's fine, there's obviously a place for this kind of stuff and I'm glad people enjoy it, but at the same time Terrifier fans think too highly of themselves. When something is trying so hard to be transgressive, it kinda negates the quality of actually being transgressive, and the whole thing just comes across as the invention of some dweeby kid at a sleepover trying to sound as edgy as possible.