This! Imho, it doesn't matter one bit what level ranges you set for the different tiers. All you need to make sure is that it's easy to enter the games at each of the tiers.
It appears to me the OP is basically ignoring 4e in his proposal, dismissing it as a failure. But it really isn't: 4e mostly suceeded with one of its design goals: It's playable in every tier.
Actually, just the opposite. I'm quite a fan of WotC's level tier system, and think it worked very well... the only reason I made changes to it were strictly economic decisions.
The biggest two complaints I saw of of the Red Box were that 1) it was not really replayable and it expected you to immediately buy all the Essentials material immediately after. And 2) the rules of the Red Box game did not actually feed into the game as a whole, so that if/when you bought the full Essentials game, the character you designed via the Red Box could not actually be used and had to be rebuilt.
So to fix these two issues I figured the following: whatever the so-called Starter Set boxed set included... it had to have enough levels in it to make it a standalone game that a new player could replay several times if they wanted (just like players of the Red Box in the BECMI days could). 5 levels seemed to make sense for that.
However... I wanted to make the game past that Starter Set a more acceptable economic decision for that player who owned it. Now that could have meant just forcing him buy the full set of books (like the 4E game expects of him currently)... but that would mean the player would be buying the first tier of material a second time. The other choice would be to follow the BECMI model and produce a second boxed set that included levels 6+.
And this is the reason why I changed the levels of the tiers. Because while you certainly could keep the 4E tiers as is... your second boxed set (Heroic Tier Part 2) would again only encompass 5 levels (6-10). To do that, but then expand the last two box sets to 10 levels apiece (for the Paragon and Epic boxed sets) seemed to be short-changing this potential new player at the very point where we're hoping to really grab him (that first purchase of his own after he's learned the game.)
Thus my decision to make the Starter Set levels their own Adventurer Tier, and then give the next tier a full 10 levels on top of that. The player now gets to play the first 5 levels of this starter game as much as he wants... and if/when he decides "You know what? I like this game! I want to continue playing my campaign!", his next purchase gives him 10 more levels. That seems a much more enticing proposition for this player to spend his money on than just another 5 levels.
So that's really the only reason for the change in levels in each tier. To give that second boxed set more levels included within it to entice the New Player to buy it once he's had his fill of replaying the starter set game.