Question When is it finally time to retire a computer case?

At a point that new needed hardware can Not be installed into it.

Also consider replacing your case if it doesn't provide enough cooling. I have an old case from '02 that has only a single 92mm exhaust fan. CPUs actually haven't increased in power use much since the Pentium 4 (up to 115W TDP!), and there are some tower coolers that will fit in that case, so a single CPU or APU could work. But we didn't have GPUs back then. A 575W RTX 5090 would surely cook everything in that case!

If your parts tend to get clogged with dust, also consider that many modern cases come with dust filters.

Older cases often came with a solid motherboard backplane, whereas newer cases often have a cut-out around the CPU area, AFAIK partly for ventilation purposes and partly to make it easier to install/remove some HSFs.

I just sold my old Haswell setup to a customer but because my current case is rubbish for 2.5/3.5" mounting, and partly I think because that old case has been with me for a long time, I'm tempted to ask the customer that when they're completely done with that computer then can I have it back

I find it easier to use old cases, including OEM cases, than ever before, because I'm seldom putting more than one, if any, HDDs in the case, nor any optical drives.

I'll cut holes for larger fans which you can do on those old cases with thick sheetmetal, adapt PSU mounts to standard ATX if it was proprietary, and more. The biggest hassle is if it's an OEM case and the rear motherboard I/O is stamped into the case wall instead of swappable I/O plate. That will cause me to abandon a case faster than most other factors, but I've converted a few of those too, just not in a long time.

I don't usually take pictures, but here's a few of a circa '99 case, pretty small for taking standard mATX motherboard and full ATX PSU:

I don't build gaming systems in old cases, and filtered intake, I accomplish by putting an intake fan on the front wall, then mount a filter panel in front of that, under the front bezel. I usually get window A/C unit filter panels from a hardware store, has enough area to do multiple systems, then hold them on with rare earth magnets, then don't screw down the plastic front bezel to the case frame interior side, so it can just be popped off to remove and clean the filter panel.

I don't have the case power switch go out often, but sometimes do add a USB3 dongle to the front, that plugs into a motherboard pin header, though these days a USB3 hub is about as inexpensive, less work, and that can be plugged into a rear USB port and allows shoving the system under or next to a desk but having the hub on the desk.

When it's under a desk, I don't care what it looks like.

On the other hand, I still need to get rid of about a dozen old cases that don't have any unique qualities to redeem them. I'll throw some old parts in, and take them to staples when I'd going anyway, and get a store coupon for recycling them.

Here's another one, showing some of the things mentioned in my prior post... cut out bezel for better intake area, put foam around to help seal cracks, a piece of filter panel, and involve magnets. The purple goop is plain old PVC cement.






Here's a '97 Gateway case, put the front intake through an added cutout to the bottom of the bezel and put taller feet on it for more intake clearance, filter panel not installed yet.



Oddly whatever plastic gateway was using towards the end of the 90's, it must have been higher quality than some, because none I had, had yellowed much and they weren't hidden from light. Same with some old Samsung printers I had, didn't yellow at all.