At that point I think most players would opt for self-made fortify marksman/onehanded gear just to make fighting high levelled enemies easier. Besides there wasn't actually any need to wear enchanted items in the first place to make the game manageable/easy. Most unique enchantments don't scale or have a high enough strength to make them worth using even if they're one of a kind. Why wear Miraak's robes when you can reach nearly 100% magic resistance and absorption in vanilla skyrim? You'd also need to grind a lot (e.g. get hundreds of extra magicka points) to make enchantments like cost-reduction redundant; it'll be unlikely that legendary skills will change preferences much.
I'm not so sure. Remember that their will be perks that reduce magicka cost. Miraak's Robes aren't powerful due to the spell absorption. It's powerful due to the tentacle burst, which has been reported to proc as much as five times
per second. I just have a feeling that very high level characters would no longer have any need for enchantments (other than chaos, but weapons are a different story altogether). Perhaps fortify one handed, archery, etc, but that's it. Players can already get weapons to insane damage without cheating, so I could even see them passing that up. I know I do. With possibly over 1000 magicka and every reduction perk, spell cost reduction enchants may simply become unneeded (again, not until a higher level). I could see vampires going for stamina regen for convenience.
Of course, I might be wrong. From the looks of it, people may simply stick with their existing armor. My point being is that certain unique enchants may retain their value at super high levels, whereas custom enchants may not.