As long as its a system like the dominion battle trophy with no ability to exchange them, it is fine, and keeps a requirement to win at floor 80/100 to get the lv 100 versions.
I'd also like to suggest that the nyzul boss key items be exchangeable for cotton coin pouches, at a somewhat lower rate of trade than the armor (2 for 20/40, 1 for 60/80/100). This would give some meaning to key items that groups might accumulate by randomly landing on a boss floor while aiming for a higher target floor (e.g. landing on floor 20 when aiming for floor 80). It could add maybe 50ish alexandrite per group if they all trade in, so it would still be less than a single salvage run, but would provide a much-needed boost in the alexandrite supply.