- Obsolete operating system - I mean what sane person would be using Windows XP or Me nowadays? Especially online.
- Old or failing hardware - I wouldn't want a 32 bit processor, IDE, no PCIe, SATA 1, Wifi G or USB 1 today. And hard drives can crap out easily enough.
Mr. Upstairs is a Commercial grade Acer Veriton M with a solid motherboard and quad core Intel i5 Sandy Bridge processor. Not the latest or most energy efficient but OK. I have upgraded RAM, power supply & video card and added an SSD so it's perfectly capable of running my train sims and keeping my wife happy. Its cooling isn't the best but I added a fan to exhaust warm air. It's making a few graunchy noisies on a cold boot but once the fan controllers get working everything quiets down. It runs Windows 10 pretty well off the SSD.
Mr. Downstairs (using it now) is an all Linux, all AMD homebuilt with a Gigabyte high quality mobo and a quad core Trinity APU. It has lots of RAM, an SSD/HDD combo and a low end AMD video card. It runs Linux Mint. It is quite fast and capable of web surfing, videos and any office work I need to do. It has 2.4 GHz wifi N which works just fine.
Both machines have nice displays, capable speakers and wifi mouse/keyboard combos. They can do anything I want today and quite frankly the only reason I would want to replace them would be if they break down and cannot be fixed economically. Neither one having a failure would put me offline because I have lots of other machines here. My wife would be unhappy if Mr. Upstairs failed but she could make do until I got something new(er.)
So it's a matter of how long can this keep goin' on? I have never been in this situation before so I don't know. I do know that some really old desktops are out there running Linux - one I gave away must be 12 years old now.