I think (i.e. in my opinion) what FFXI did with the Warlock Armor, Duelist Armor, Warlock Armor +1, Duelist Armor +1 is an example of how to do high-level gear progression properly.

Have both sidegrades (Warlock, Duelist) that are equally powerful, but in different ways. But also have upgrades to those sidegrades (Warlock, Warlock+1) that require the original pieces. That way, a player's effort and time in getting the original pieces are not wasted when the new gear becomes accessible. They become an investment.

Just my opinion.