I have regretted taking apart one deck in the 60+ decks I have ever taken apart and that was Karador. It was my first ever deck and it has come a long way since its very first incarnation. I took it apart because I had a Jund deck and then merged the two to make a 4 color deck and that deck was awful. I ended up taking that deck apart entirely and then about 1 year after that, I put Karador back together. It is a staple deck that I don't think I will ever dismantle again.
As for the rest, I concur with other posters that the reason I take decks apart is because they just weren't fun. Either the execution was flawed in some way where the deck didn't perform nearly as well as hoped or, in some cases, it was just too oppressive for my group so no one was having fun with it. And, in other cases (and why I don't currently have a Jund deck) is because it plays too similarly to another one of my decks. Every Jund deck I build always just feels like a worse Karador knock-off. I will sometimes revisit an idea, as I did with Riku, but if it doesn't work then it doesn't work and I will take it apart and try something else.
Throw another voice on the pile of deck fun matters. It's a tricky sweet spot to try to land - you've gotta like it, it's gotta be good times to play, and your playgroup can't hate it too much. Any one of those goes to hell and the deck becomes jeopardised. A nontrivial amount of paper happens because of either of the first two going wrong, and then the deck lives in a drawer until I need something for a different build. The only times I feel sad these days is if I piece together a list that I'm fond of, but find out my opposition hates. I rushed Bruse Tarl, Boorish Herder + Tymna the Weaver because the shell was a riot to pilot, plus come on, Wei Scout unironically used in EDH. Unfortunately, it was just greeted by sighs and groans, in spite of me openly trying to emulate another group member's build style. I tried to learn from my mistake, and didn't force Grothama, All-Devouring into paper through similar groans from Cockatrice testing.
A deck I wish worked out was my Xantcha, Sleeper Agent draft. The meta didn't mind, probably because I always felt like I was a few turns behind what everyone else was doing. The only way to get the deck to stop sucking would be to look into wrath crawl'y elements that the foes would hate, so I didn't even bother. Tricky one to balance, as mentioned.
There's been a few I've regretted, but there's usually a reason the deck is being dismantled. My Thraximundar was tremendously janky but lots of fun to play, so pulling that apart recently was a little sad. Still, most of the parts are still around, and it's never really gone while you remember the fun times you had with it