It's one thing to lose on champion select because your team individually clicked LOCK IN for an inferior team composition, and the rest of the team didn't dodge as a result.
It's another thing when you are last pick, your pick is taken, you make a last second champion change, you go look runes up, take half a second too long to confirm changes... and you get the default inspiration rune page instead, and that's basically lane over if you're a laner and game over if you're not. Similarly if you forgot to swap smite from your previous match (though your team is able to warn you about it).
Runes and spells you don't lock into; those things you're locked into. They can lose the whole team games in entirely preventable ways. Yes, it's true, you can play around your surprise runes and spells (consider that actually seeing your runeset is hidden behind a fairly obscure keybind or bit of the interface). But, this is much more commonly the source of tilt and frustration and inting and raging and flaming.
It really doesn't have to be this way. So long I doesn't leave the podium, please let me change runes and summoner spells from the shop. If it _really_ needs be, if you _really_ think that this sort of mistake should have some form of in-game consequence, make it cost 25 gold to change spells and 25 gold to change runes so you end up behind on potions instead of being behind on stats.