I don't expect to be able to upgrade the graphics card or the CPU but swapping out the harddrive or adding more memory used to be relatively easy for iMacs. Certainly not impossible.

What if in five years your harddrive crashes and they've already switched to yet another proprietary temperature monitoring standard and no longer sell drives equipped with the old system?

I've been using the same iMac for 5 years now (and upgraded the disk to an SSD about 6 months ago). Not being able to do that with a new one is a deal breaker.