Thus far, I've unloaded equipment for any of these reasons:
1) Something you don't use.
2) 'There is a better <whatever> that can better handle what I need (low light shooting, in my case).
3) Anything malfunctioning.
Maybe 35 years ago, my father told me that if I haven't used something in 2 years, it's almost guaranteed I never will. He was right! When I finally moved from a 'compact' camera (fancy point and shoot that I could shoot in manual mode, etc) to a DSLR, the compact became surplus so it got sold on
ebay a week or so after I got the DSLR. As my photography skills improved and, at the same time, most of my photography transitioned to indoor events, I had no choice but to upgrade lenses as well as camera body. At that time, I hadn't decided on whether I wanted to use mostly 'primes' (single focal length lenses) or zoom lenses. So, I decided to try 'both ways' (I had the money to do so back then), settling on 4 primes and 3 overlapping focal length zooms. After maybe 8 months or more, I found I used the zooms perhaps 90% of the time and primes the other 10%. Of the primes, the 135mm f2 was also 90% or so of prime usage, the others, maybe a couple dozen frames apiece in each of multiple shoots 'just to see how well I use them'. I ultimately sold the other three primes from disuse. For what it's worth, back in my film-shooting days, I had mostly on zooms and only the 50mm flat-field prime I bought with my first SLR.
As I transitioned to mostly low light shooting, it didn't take long to decide I needed to upgrade my camera and get faster (lower f-stop number) lenses. That was at the same time I did my 'prime or zoom' tests. I also needed a camera capable of shooting at ISO 2400-3200 so I could raise the shutter speeds to reduce images blurred by subject movement at shutter speeds in the 1/10 to 1/60th or so. So, I ended up with 2 f2.8 zooms and one f4. A bit over a year later, I upgraded the body again to one that I'm OK with shooting at ISO 6400 or higher when needed, and ISO 25,600 when I'm in almost total darkness. Yea, at 25,600, the noise is extreme, but I've been able to get acceptable results after fooling with noise reduction in Lightroom and Photoshop Elements (I'm a hacker) for 5-6 minutes per picture. The Canon 5D mark iv is 'calling me' for it's improved high ISO performance, but money's tight these days.
One of my zooms was the Canon 24-105 f4L lens. After 5 years of using it, it malfunctioned due to it's well documented design flaw. Out of warranty, it went to Canon and it was all fixed. 8-9 months later, the same problem happened. Off to Canon again. I used it for only 1 or 2 shoots after I got it back and the lens error occurred a couple of times, I decided I didn't want to put up with the risk of another total failure. Canon also produced the mark ii version of the lens as of the last year or so, so it gave me the option of buying the better lens which I finally did. I sold the first 24-105 on
ebay and fully documented the lens' history in the listing so the buyer knew up front what risks he was taking. It's been a couple months now, and he left positive feedback for me. Win-win in my book.