Look at how long it takes them to update one, single model and write some new abilities for it. An amateur team could do that in a week or two, yet it takes Riot a year or more.
Amateur Riot use to crank out champions at a rate of 1 or 2 every 2 weeks. Turns out it was too fast to maintain, and a lot of the champions developed at that time are ones Riot's had to update or rework since because the designs just didn't last. It became a Quality over Quantity trade off, and they opted for quality.
But we get new skins every patch just about, and they don't take years to make those.
They also don't need to build a new kit for those. The team that works skins is separate from the other teams too, so it's not like their work slows down anyone else.
Look at Yasuo / Zed / Ekko / Vayne / etc., being near permaban / highly played (with absurd winrates), but not being touched.
Zed went from 90%-ish pick/ban to 40%-ish pick/ban with no balance changes in that time. Ekko they were constantly changing trying to get him out of Tank Mode, Vayne's been considered weak several points this season, as has Yasuo.
Your winrate / playrate stats also need a fact check, because none of it holds up for patch 6.21
Look at the terrible coding all throughout the game that Riot outright refuses to fix (that Jayce hammer bug? They actually refuse to fix it)
That cosmetic bug?
Ignoring the countless other game impacting bugs that get fixed (like Braum getting 100-0 by Baron with shield up, Aatrox being unable to cast spells, Xerath crashing the game) and the underlying tech improvements (such as the projectile system overhaul and doubtless others that are less pretty).
Lets also ignore that they basically rebuilt your internet so you could play with less lag, something no other game company has done, F2P or not.
Your complaint sounds more like "I don't know what's going on in the background, but fix what I consider broken now before you handle other things".