If these are not for public display, which I assume they aren't, they can be patched, darned, and/or have sections cut out and replaced. Actually it is possible to do all this for outer garments too, but requires a little more finesse and depends on the amount of damage.
I make my own leggings from merino wool sweaters; the pair I have on now I sewed completely by hand, but I have also used my machine. A zigzag stitch is best for stretchy fabrics. I always keep scraps in order to make repairs later :)
In your case, I would decide which were the best candidates for repair and use the others to harvest material for making those repairs. Small holes can be darned or patched. To discreetly patch a small to medium hole I first trim any loose strings, then I will take a scrap piece a little larger than the hole and put it on the wrong side of the garment and taking very small stitches, sew it by hand all around using a little bit of unravelled yarn from the scraps. I will then sew small running stitches up and down the patch (side to side works too, but doesn't blend in as well as up and down). These are like quilting stitches or basic sashiko stitches so the whole thing is firmly anchored and the edges of the hole are all tacked down against the patch. If the area around the hole is thinning, I'll make the patch large
enough to cover this whole area.
I'm uploading a photo of my old pajama bottoms to try and show what I mean. I've worn these for about four or five winters, but have got to the point where I'm just sick of mending them, and have made a new pair--these actually aren't merino, just regular wool. I didn't care that the mending on this didn't match, as I don't even wear them around the house--just upstairs in bed. Most of the patches are actually scraps of felted cashmere which feels makes these so warm and soft on the inside.
To replace an entire area like a crotch, you can cut out the old section and use it as a template for cutting a new piece. Or cut out the crotch of one of the "discard" pairs and use that for a template. The new one can be all in one piece or can be in two or more pieces sewn together, depending on your preference and how much material you have to work with. I advise basting in the new piece by hand first and seeing how it fits before machine sewing it in, as ripping out stitches in stretchy wool is not fun. The edges of the new section
should be zigzagged or overlocked to prevent any unravelling. When sewing by hand I use a running back stitch to sew them together and blanket stitch to finish the edges.
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