Look at the last few years of champ releases. Riot is bad at it.

Let's look at Riot's history of champion releases and evaluate them for quality. Note that I'm not talking about how strong or weak they are, but rather how healthy they are for the game in terms of kit and the ability to be balanced, taking into account popular opinion as well as Riot's own determination.

Kha'Zix: invisible one-shots, only allowed to be (too) strong because of "esports." Elise: an unbalanceable mess that was nerfed into the gutter until this World's patch because - and I quote - Riot wants to see her "dominate" again. Zed: not too egregious, but it seems like he's balanced around being just as hard to play as he is to play against. Nami: probably one of the most reasonable champions ever made, but Riot's nerfing her now because of "esports." Vi: a history of doing too much damage while building tanky, but her kit isn't actually terrible. Thresh: nerfed into a shadow of his former self because he's too useful, especially in coordinated play. Quinn: rushing a product out the door because you value a deadline over quality is sad. Rationalizing it with "we hit quality" (translation: "eh, good enough") is shameful. Zac: Riot has no idea what to do with him. Lissandra: Up for a rework, a passive that's just "your mana costs aren't as high as they look," etc. Aatrox: everyone could tell within a month of his (original) release that his kit was trash and would need a rework. It's finally here, and I'm not sure it's okay. A tentative "maybe." Lucian: he's had some troubles with being OP when the meta favors him and forgotten when it doesn't, but it seems like a reasonable design. Jinx: I don't see a problem here. Yasuo: sigh Vel'Koz: good job on this one! Braum: a bit too good at shielding a teammate, which is probably why Riot doesn't want him in Worlds. Gnar: three Azir: nightmares Kalista: in a row Rek'Sai: so inherently OP that her defining feature, a map-crossing ult, had to be reworked into a mediocre dash. Bard: I have no idea. Ekko: seems okay, now that he has to actually build damage to do damage. Tahm Kench: the only champions that are balanced around occasional untargetability are the ones with such an ability in their kit or mages who can build Zhonya's. Creating a support who can bestow this status on anyone was a tremendous mistake, which is why Riot doubled Tahm's W cooldown. The lack of foresight is really disappointing. Kindred: is either the best jungler in the game or the worst. Also has a habit of providing a Tryn ult to champions whose balance doesn't account for that possibility. Illaoi: pretty fine. Jhin: who thought it was a good idea to mess with several fundamental game mechanics? Now we have a champion who takes ordinary things like "sometimes you hit harder" and "this increases your overall DPS" and turns them into THE FAST. Aurelion Sol: I don't think his kit is unhealthy per se, but it's not popular, either. Taliyah: zooms across the map to CC someone across a minefield so they die in midair. If she didn't have a game-breaking bug she'd probably be pick/ban in competitive play. Kled: I don't hear anything about Kled, and I'm not sure if it's because he's fine or because he's been nerfed into oblivion. Ivern: a champion who breaks fundamental gameplay assumptions (notice a trend?) like where brush is, being hit by jungle monsters, and needing to spend time at a jungle camp instead of clearing it instantly. Goodbye, happy tree. Camille: as it turns out, overloading a champion's kit with a ton of extra crap makes them impossible to balance. Who knew? Well, pretty much everyone, but no one who works at Riot. Rakan: seems okay, but his performance is tied to Xayah's. Xayah: way too strong with old Essence Reaver, and seems to have disappeared now. This goes beyond "favored by the meta" into "only viable with a crutch that no longer exists" territory, which indicates a problematic design. Kayn: seems really strong all the time, but I don't know that his kit is actually toxic, so he gets a pass. Ornn: oops, he's too useful, gotta keep him in the gutter with his unbalanceable friends. Zoe: released not in spite of but because of the fact that she makes people rage, which makes them more likely to stay addicted to the game. "But hurting people is profitable" is not an ethical justification for any game design decision and whoever thought this was okay should be absolutely ashamed of what a terrible human being they are. Kai'Sa: she has no CC and nearly no team utility, but she has so much damage and so many ways of dealing it (AP, AD, on-hit, crit). To be honest, I think this is actually good, because she doesn't have to rely on a single crutch item that makes or breaks her, like Aurelion Sol with GLP or some ADCs with old Essence Reaver. She's strong, particularly as an "exciting" champ with Worlds coming up, but I don't think her kit is bad. Pyke: "healthiest kit ever" for a month or two, but now somehow unbearable to deal with.

37 champions in the last six years. Two designs I don't have enough information on, six good designs, nine middling designs, and twenty examples of how not to be a game designer (many of which are recent). Riot hasn't released a new champion in four months, and I hope that trend continues, because they're actually terrible at it and don't even realize.