If you're level 5 and getting paired with level 25s, it means you're most likely a smurf and have been ruining hundreds of legit level 5s days to the point where your hidden normal MMR is beyond higher than what it should be at that level. People may argue "oh but that's why they should just only do co-ops", but even that gets boring after repetitive grinding on the journey to level 30. Once in a while people will normally want to play with real humans. A new player shouldn't be forced to choose between either 100% roflstomp smurfs or boring co-ops grinding. They should be fighting other legit beginners in normals. This factor alone would already discourage a new player-base from forming. Look at what happened to Star Craft. If you play vs. Humans typically it's just smurfs waiting to get their joy of the day by roflstomping new players.
On another note, I think if the overall Team level average exceeds the locked summoner spell, it should be temporarily unlocked for that game in particular for an even playing ground. Seeing we've established Level 5 smurfs are being paired with level 25+ with Tier 2 runes/Masteries and all spells unlocked, I can see where the OP makes his point.
I think all Tier 1 runes should be temporarily scaled up to Tier 2, and Tier 2's temporarily Tier 3's if the overall average Team level exceeds level 10 or 20. I also think once a player hits level 10 the Tier 1 runes permanenly upgrade to Tier 2 automatically and the Tier 2's permanently to Tier 3 at level 20.
This won't really stop the rune slot unlocks but, if we removed that then the feeling of reward and unlocking kind of go down the drain.