Check out the Corridor Crew episode on it. A lot of what we love was actually due to budget limitations! Like the super detailed backgrounds are digital paintings as opposed to being fully 3d.
They cut a LOT of corners but not in a bad way. Limitation breeds creativity and Riot absolutely nailed it
Well yeah, the line has to be drawn somewhere, but it's still super high budget compared to any animated series out there. They spent roughly 10 million dollars per episode! High budget anime would usually spend something like 1 million dollars per season. One punch man season 1 reportedly spent $80k per episode. The difference is massive.
It not being fully 3D doesn't mean that it's of lower quality.
For example when consoles got powerful enough to draw 3D backgrounds you could see a significant decline in background quality, since you can have a higher quality 2D background for a fraction of the resources.