You're opening a box which you should have left closed.
First of all, you DO have to buy champions. You can't seriously pretend someone can just use free week champions all the time. More champions means more versatility. Now, of course, you can buy them with IP, but since you guys decided to make every new champion cost 6300 ip and reduce IP rewards... On top of that, runes can only be bought with IP, which means buying champs with IP essentially gimps you unless you already have full rune pages.
Unlocking a single 6300 ip champion will easily take around 50 hours of gameplay. Pretending you don't mean for people to purchase them with RP is laughable, and pretending that it doesn't affect gameplay is absurd. And inevitably, if you keep this stance, you'll get compared to DotA 2, which is also free, but has all champions unlocked at the start and does not have a masteries/runes metagame that requires grinding. Their income comes entirely from cosmetic items, and know what? It works.
If the IP system was reasonable, there would not be an issue. But it isn't. IP gain rates are ridiculously slow unless you buy boosts (which cost RP, btw), which barely gets them into the "acceptable" range. Unless you spend RP on boosts and champions, you will remain crazy gimped for a long, LONG, LONG time.
It's like claiming someone working minimum wage can become a millionaire with that job. Sure, if he lives to be 600 years old, never has any accidents that force him to spend money and works the whole time and lives extremely poorly. Realistically though, it's a dubious claim. You'll buy the old champs at 450 ip and even then it'll prevent you from getting runes for a while. And let's not even count the cost of additional rune pages.
I agree that runes and masteries are a bit too grindy right now. I think there should be some sort of reward/discount for players just hitting 30 to help people become competitive and purchase their first set of runes and have enough champions to make a draft (unranked) team.