With same cost, the same players at platinum can keep playing against the same playerbase which over time rises into Legend, playing exactly equally difficult matches in terms of ability to gain crowns.
Yes, maybe more intensely grinding players tend to be better, but thats no different from simple mage level categories.
Maybe the relatively big cost subtraction will differentiate the speed at which differently skilled players rise in a season, but it's still directly multiplied by time played which means that effort can be used to replace skill, and that the plat-legend range is quite heterogenous mix of differently skilled people.