Honestly, I'd have agreed with you in the past... When the old rune system existed, I purchased rune pages until I had either 16 or 18, I can't remember. Ever since the new system has hit though, I literally only use one page(Even though I still own those other 15 or 17 or whatever). I haven't even CREATED the other pages with the new system. It's so quick to just change my runes as necessary before the game to fit what I need in that specific situation.
The only game mode where you should be playing something that you didn't expect to play/have no exprience with is ARAM, and even there you can very quickly throw together a basic Dark Harvest page that will work with almost any champ on ARAM(Or a few other "Blanket coverage" pages, for tankier champs or whatever).
In normal game modes where you actually pick your champion you should at least have some idea of what you're doing before you pick a champ. Knowing which rune page to pick if you have 20 means you should also know which runes to throw into a page for that champ - Which I often do in 15seconds or less during the lobby.
Having more pages is (Presumably) nice for some people, and helpful.. But the current system is only 9 slots/choices to make, so it's very very very easy to just change them during the lobby. Not knowing what ones to pick means you also wouldn't know which page to use if you had 20 pages, so having more wouldn't exactly help you there either.
Practice it a bit, get in the habit of changing your runes manually before each game during the lobby. Once you get used to it, it'll help a lot without having to spend on extra pages. Personally I open my rune page the moment I lock in my champ, and select the "Most Likely" set of runes I'll want to use, such as Electrocute/Sorcery secondary as Diana, while saving the last stat rune(The defensive one) to choose after my lane opponent has locked their pick, so I can choose the relevant defense. While waiting I'll have in mind "If they pick x then I need to change my runes to z", so for example if I lock Diana and they pick Swain I'm going to want to switch from Electrocute/Sorcery to Phase Rush/Domination. It takes a little getting used to, and you DO need to know what runes to pick.. But again, if you don't know which ones to pick on the spot, you also wouldn't know which page to pick if you had 20 pre-made. It's only 9 slots to fill, it takes 30 seconds or less once you're used to doing it. Just open it once you lock your pick, and keep it open ready to edit if the enemy team throws a curveball at you, and save it/close it after you've seen the enemy team's picks and chosen your relevant runes.
