I generally concur with the “keep it until you sell it” philosophy - space permitting, of course. Custom made, perfect fit, ease of shipping/resale, etc. Between studio and stereo and HT gear though, I’ve had so many boxes, that I had to keep most of them in the top of my garage which is not enclosed.
One winter a family of squirrels moved in without my noticing. By the next time I checked, they had an interconnected condo development up there with holes going through multiple boxes so the kids could have their own bedrooms, etc. So I had to trash a huge number of them at one point.
And now I’m moving, so I’m going to miss having some of them. The primary one being the one for my VPI Scout. Though it won’t be the end of the world to separately package the plinth, motor, platter, arm and cartridge. Mostly solid stuff.