TL;DR - Riot is a company that needs to make money to operate so they can't be giving away ~400RP a week per player for winning a game
Introducing mastery points for little lege... well, that actually seems like a weird thing to gather points/ have a 'grading' system for. Sure, you could get a different amounts based on placements but there is a ranked system they already have. Plus, people have different amounts of little legends so then some would less mastery available to earn and cause players to complain about... how to put it, (references 90% of 'new thing'-posts comment sections since the first event pass) the lack of things not being free.
With the current ranked system, you can learn about the game, get better, and climb the ranks feeling happy about self improvement. And at this point, we can only hope to be rewarded will come at the end of the season, like other ranked game modes.
Also, sure, maybe one day they'll add a chest that we could get once-a-week from high placements in games or maybe even use the current mastery chest system they already have available depending on someone's preferred game mode making it your choice where you earn it. I feel like winning a tft game is pretty similar to a party member/ yourself getting a s- or better, so seems pretty fair.
This then leads to the problem of the type of loot available in the chests (as a SR champion skin shard would not do much for a TFT only player), which would lead me to believe that having separate types of loot would be better.
For now, we shall wait, as the game is still in development and Riot is trying to gather a bunch of information right now, so we can only wait as the see how everything else progresses. I get, you may not like the fact that some other dude has a sparkling, little dude that moves around, but what's so wrong with your little moles/bird/stone man or blob? They're only cosmetics after all.
BUT...
Ahhh yes, Riot is currently making NEW little legends already at the same time the game is being developed, BUT they are in fact still working on the game too, hence weekly patches for balance and fixing some bugs.
Anyway, let's see what this looks like compared to current crafting system. There were hundreds of skins available and years of gameplay and spending before hextech crafting existed, capturing the audiences dollar for those interested, and then working towards a rewards system later on. By implementing this system immediately, it would decrease the value for those who pay for their cosmetics once there were a chance for everyone to get a reward as such once a week, and people would be less incentivized to buy things. Nothing wrong with that, except from RIOT's point of view, because they, as a business, have to make money to keep the game going. No one really knows how much this amount should be. Start your own game with hundreds of thousands of a players and let us know, lol
As a community we shouldn't rule out the fact that RIOT is a business, and if you don't like what or how they're selling, just don't spend money. cause at the end of the day, everything we're complaining about are just things that makes the gameplay look nicer.
And if people want to get specific about how much money they lose and the "ridiculously long' amounts of time it would take to get to certain points; here, I saved you some time:
108 weeks with the current 6 Lil' Legends (x 6 styles of each x 3 stars) weeks assuming everything drop rate was equal (which it's not so it'd be faster) to get EVERY little legends to tier 3, AND assuming there was strictly little legends in the chest, which probably isn't entirely accurate either. Also 73 weeks at the absolute max. for a single 3-star (6x6x2 stars each + 1 next chest would HAVE to be a 3rd appearance of one of them)
Sure all of this is a lot of time, but think about it this way: Every player who plays once a week minimum, should be able to get every TFT little cosmetic in the game _ as-is currently in 9.15_, for free? They're developing more options, making all these numbers less likely as more comes, I get that, but what people are on about right now, is that you should play minimum 52 times a year and get $135 of loot, or 490RP (let's round down even to 400RP), for winning a game once a week. That's not how business works.
Disclaimer: I know I'm the one who suggested the once a week loot thing in this post, but I would like to point out that I don't see that as being reasonable at this current point of the game. I was hypothetically comparing it to the current chest reward system, in context of what it seems people are suggesting. If not, my apologies. Maybe some sort of shards, like skins would be more accurate, to which you'd just divide the aforementioned rewards by 4 (so, about 100/125 RP per week, or in turns of the best value purchasable equivalent is about ~$35 a year) which is more reasonable, but remember, it's also free and the scale of the playerbase.