They expressed champion sales (for RP) was/is actually a sizable portion of their revenue when they were doing runes reforged. Since people would no longer need to spend IP/BE on runes, people will be more prone to buy champs with game currency instead of buying them for RP, so the change to make runes free did hurt them financially.
This is more or less the reason they expressed for why they were net lowering the amount of orange essence people got for disenchanting stuff (we get more stuff to disenchant from events and so on, but even accounting for that it's somewhat less overall). So champion sales are actually important enough that they felt the need to address OE rates much to the dissatisfaction of regular chest buyers, so I wouldn't blow them off just because they're not as fancy as skins.
Incidentally, these days we can get any paid content for free in theory now due to hextech crafting, so really the games the free-est its ever been.