Ina, the toe-up socks with hourglass heel I make - being the basic pattern from the Twisted Sisters Sock Workbook - does allow you to drop out heel or toe and reknit. I haven't done it yet, but the book explains how to unpick one row, hold the rest of the sock there on dpns, and then knit away from the dpns. In the case of the heel this is then just like adding an afterthought heel, and you join it in using Kitchener stitch when you reach the other side. With the toe you either knit a toe-down new toe, or if you want all the stitches to line up then you'd knit a new toe from the tip and graft it in using Kitchener stitch.
So far I haven't given any of my socks enough wear to need to replace either toe or heel - and nor, amazingly, have I yet needed to repair BH's first pair, which has been a big and very nice surprise, as he wears them pretty much all the time he's off the farm - but I would be interested to know how you reinforce your heels, Ina? I love wearing my homemade socks so much I do want to use them for everyday use about the farm, and for sure the heels will then take more of a pounding, as you say.