Some is sold on the used market; some is destroyed. There are plenty of used V100 and A100 available now for example.

I used (relatively) ancient servers (5-10 years in age) because their performance is completely adequate; they just use slightly more power. As a plus it's easy to buy spare parts, and they run on DDR3, so I'm not paying the current "RAM tax". I generally get such a server, max out its RAM, max out its CPUs and put it to work.

Same, the bang for buck on a 5yo server is insane. I got an old Dell a year ago (to replace our 15yo one that finally died) and it was $1200 AUD for a maxed out recently-retired server with 72TB of hard drives and something like 292GB of RAM.

Just not too old. Easy to get into "power usage makes it not worth it" for any use case when it runs 24/7

Seriously. 24/7 adds up faster than most realize!

The idle wattage per module has shrunk from 2.5-3W down to 1-1.2 between DDR3 & DDR5. Assuming a 1.3W difference (so 10.4W for 8760 hours), a DDR3 machine with 8 sticks would increase your yearly power consumption by almost 1% (assuming avg 10,500kWh/yr household)

That's only a couple dollars in most cases but the gap is only larger in every other instance. When I upgraded from Zen 2 to Zen 3 it was able to complete the same workload just as fast with half as many cores while pulling over 100W less. Sustained 100% utilization barely even heats a room effectively anymore!

Wake on LAN?

Then you cannot enjoy some very useful and used home server functions like home automation or NVR.

The one thing to be careful with Zen 2 onwards is that if your server is going to be idling most of the time then the majority of your power usage comes from the IO die. Quite a few times you'd be better off with the "less efficient" Intel chips because they save 10-20 Watts when doing nothing.

A similar one I just ran into: my Framework Desktop was idling @ 5W more than other reported numbers. Issue turned out to be the 10 year old ATX PSU I was using.

Maybe? The price difference on newer hardware can buy a lot of electricity, and if you aren't running stuff at 100% all the time the calculation changes again. Idle power draw on a brand new server isn't significantly different from one that's 5 years old.

To be clear, this server is very lightly loaded, it's just running our internal network services (file server, VPN/DNS, various web apps, SVN etc.) so it's not like we're flogging a room full of GeForce 1080Ti cards instead of buying a new 4090Ti or whatever. Also it's at work so it doesn't impact the home power bill. :D

In-house hyperscaler stuff gets shredded, after every single piece of flash storage gets first drilled through and every hard drive gets bent by a hydraulic press. Then it goes into the usual e-waste recycling stream (ie. gets sent to poor countries where precious metals get extracted by people with a halved life expectancy).

Off-the-shelf enterprise gear has a chance to get a second life through remarketing channels, but much of it also gets shredded due to dumb corporate policies. There are stories of some companies refusing to offload a massive decom onto the second hand market as it would actually cause a crash. :)

It's a very efficient system, you see.

Similar to corporate laptops where due to stupid policies, for most BigCos you can't really buy or otherwise get a used laptop, even as the former corporate used of said laptop.

Super environmentally friendly.