So far as your desktop, you can certainly upgrade your computer without it being more cost effective to do a whole new build. It really just depends on what you need. Mostly it comes down to the limitations of your motherboard.
Every so often they changed the CPU socket, so if you need a new CPU and you already have the best that socket can do, then yeah you’re maybe looking at a new build at that point anyway. But otherwise you can just get an upgraded CPU of that socket. Similarly, eventually your motherboard won’t be able to support the latest version of RAM and if you need that you’ll have to replace the motherboard. So on and so forth.