You know there is a reason, there are low priced IP champs. A. They were made first so they have simple kits and B. They are meant for people to learn the game.
If we had mechanically advanced champs for starters, or if people were to play them as starters, we'd either have people giving up on the game or being frustrated by the (annoying champs at low levels) like Zed, Fizz, Nidalee, etc.
You also mistake the principal of having to earn something with being privileged with having something. IF there was no IP system, there would be no satisfaction from winning games. "I spent an hour worth of my time playing a game, what do you mean I don't get a reward for winning?" From a gamers point of view there Is effort spent but no reward. It'd be like playing a strike on Destiny and not getting any sweet loot. So what's the point of playing?
IP is also how they separate champ players. The reason they release the new champ at 7200/7300 is because they want the players who have played the game long enough to get that much, to give them the best feedback. Champs that are old are fairly well put in their balance (Most of the time) so they don't exactly need the help of long time players to help them give feedback (necessarily).
TL;DR- IP is how they separate the new players from the experienced players. Those starter champs are meant to be used first since they're easy to use or easy to fight against. Mechanically advanced champs require more of a knowledge of how League works before they can be played. IP is also what they use to reward the player. If we didn't have any reward for playing a game, why would we?
On the other note, I do wish they would implement a system like Dawngate's, where doing very well (with a bit or RNG) could lead to their equivalent of Runes and occasionally a new champ (Shaper). This rewards the player base heavily and feels a lot more satisfying when doing very well (Unless Rng says F**k you, since it will) in a game, and where being honored (In their terms Karma) could contribute to getting a reward.
That's my two cents.