I sold my mtb earlier this year because I didn't use it much at all. It didn't fit great and i didnt benefit from it on the singletrack I ride.
All the singletrack near me is twisty river bottom woods. It's flat, twisty, and heavily wooded so a slack mtb with absurdly wide bars that acts like a humvee isn't what i want. I prefer a nimble gravel bike with narrower bars that dont get stuck between trees.
Admittedly, I dont roll over trail logs like i could on my mtb. I can clear some, but not the larger ones. I'm ok with that.
If I ever find an mtb frame that is for the cost I want and has geometry that isnt super slack, ill build up something for singletrack with wider tires than my gravel bike. I doubt I'll be faster or have more fun.