I agree. For a long time now, I considered the CPU, MB and RAM a matched set due to the CPUs or MBs having such an assortment of memory controllers. If either dies, finding a good replacement years after it was made is not so easy. The only thing this makes hard is making good systems from bad parts. For example, I have a 939 MB for a HTC, but it was underpowered. I just happened to have a dual core Athlon64 from a system which the MB fried. So, a quick swap and my HTC went from usable to awesome. That part of the hobby will be gone someday, but that will still be years away.
I often need to tell clients it is time for a new computer time when a CPU (clogged cpu fan) or MB dies. Simply, you are left finding used or refurbed parts which only offer a lateral move in performance or will suffer the same fate of the original. At times, you end up needing to add a video card if the new MB doesn't have one built in or the RAM configuration may need to be different due to the part you replaced. Or, you find out both the CPU and MB are dead and then you certainly need new RAM. So, unless you have limitless piles of spare parts to swap in the cost and time will not be worth fixing it when for the same cost you can get a modern computer with a lot more power and an OS upgrade to boot. Fixing the only the RAM or the drives is usually cost effective, but replacement of the CPU/MB is just a big pain unless I give away my time for free.