"We took a look at how other games have tackled similar systems to see what felt good and what could use some improvement. Existing systems like Counter-Strike: Global Offensive’s loot boxes for StatTrak weapons and Dota 2’s subscription model for Hero Relics showed us that rarity was a core component for these types of systems. As a result of what we were seeing, we initially designed an early version of Eternals with a loot-based system, but it felt overly complicated and too punishing to be a satisfying approach for League. As a result, we went back to the basics: We’d package sets of three Eternals for players to get with RP."
This is why Riot games has been in the shitter for these past few years, they've been doing nothing but copying from other companies to get a penny in their pocket, instead of trying new things out for themselves and call League unique. Hell, the entire game was mimicked off of Dota 2 for fucks sakes. Knowing some players though, they're just going to buy it without thought because it's new and they wanna show it off to all their buddies.
This is way less of a cosmetic than a skin is, which I can understand buying for. You shouldn't have to pay for literally ranking up your favorite champion. People have been asking for a mastery level 8 thing for a while now, and you give them this.
This is why we don't ask for things from Riot anymore guys, because when you do, they pull the "For just $19.95, you can get your very own personalized achievement list!" It's nothing more than a way to rob your pockets.