Some people, like me, spent a ton of real money to unlock all champions faster. According to Riot, it's not "necessary" to have all champions unlocked. However if you don't own a champion, you cannot pick that champion early in the team drafting phase for someone else to play (swap/trade) - even in ranked. It is a disadvantage, though admittedly minor - a 5 man team that owns all champions has more flexibility in drafting than an enemy team that has holes in what they own.
With enough play times, you'll have more than enough to buy all the heroes without paying real money and then there's nothing else you can use it for.
Real money is only required if you want to buy skins.
Also, with hundreds of available champion to play you are not really required to play the most recently released champion. Just play from pools of all existing champions until you get enough point to buy new champion.
This means that if you set a limit on it, you'd be limiting your profit by an extreme amount.
I'm all for it by the way, I think games that sell game affecting content for thousands of dollars are targeted scams portraying as games to get away with it.
Especially because games like league of legends and path of exile had shown how you can do free2play while only selling cosmetic items, if you make a good game.