I do feel like the reward to effort ratio of older S ranks has gotten a touch out of whack due to the addition of new items to be purchased with allied seals which effectively makes ARR S ranks more valuable then HW/SB ones.

I don't think it would be unreasonable to at least implement some kind of HP/damage scaling. IE, they could give "outdated" S ranks a stacking buff that reduces damage down to a small fraction of normal and only loses stacks from having same leveled or higher players in their vicinity to discourage people from soloing them at least.