Please don't call me ignorant and start attacking me when I've done nothing of the sort to you. I could easily attack you on your clear lack of knowledge on basic economics, supply/demand and cost/benefit analysis but I'm not because I was under the impression we were having a friendly conversation.
Again, I'm not sure what's confusing here. You're comparing cosmetics to gameplay. SR can be pink, it can be red, it can be normal SR, it can be your shadowy noxus arena, anything like that. It can be any theme, any color, any ambiance, you name it. Nothing has changed. It's the same exact champions, playing in the same exact lanes, farming between 6-7 minions every 30 seconds or so. Objectives are in the same places. Nothing has changed. It just looks cool.
When you rework champions, you encourage players that don't have the champion, to buy the champion. When they buy the champion, they have a good chance of buying a skin.
However, let's pretend everyone owns this champion, right? Let's say they rework Master Yi. I assume most players have Master Yi, so why rework him? Because those that don't play him may pick him up and also, buy a skin.
Not only does reworking a champion bring direct champion sales and skin sales, it also brings good publicity. Now here I admit, your re-color idea is about the same thing. If they put work into constantly changing the theme, yeah, League might get a cool rep for having dynamically themed maps as the year goes on. But it's not the same as a champion. When there are balance issues and player behavior issues, people won't see changes in themes and suddenly want to play again or start playing because, even though the core issues haven't changed, the rift looks cooler.
However, when you're reworking champions, you're not doing it for fun. You won't see a Kled rework or a Xayah/Rakan rework anytime soon. You rework champions that don't fit the game anymore and are weak, or their kit is unable to be balanced with numbers and needs an overhaul. Poppy, Taric, Galio, Urgot and Aatrox soon. These are all champions famous for being absolute shit (Urgot not so much, but still considered pretty bad). By reworking them, you're reintroducing content that wasn't be used and therefore going to waste. You show the players that the game is always changing, that ALL champions are meant to be played, and if not, we will change them so they are. It may take them a long time, but in the end, they're changing and advancing gameplay. The theme of the rift isn't gameplay. That's purely cosmetic. That's the difference.
Any questions?