Lord Housewife, your OP would be vastly improved if you gave some examples to support your thesis, rather than just stating your thesis and assuming its truth, and then spending the latter 3/4 of the post saying...well, nothing interesting.
Let's take Riot's S3 itemization changes as an example. How, in your opinion, did they make changes that strengthened that which was already good, as opposed to strengthening champs that were not good before the patch?
Because I look at Black Cleaver and see a rejuvenation of overlooked champs like Talon, Pantheon, Urgot, and MF (to be fair, this comes along with champs like Riven, Renekton, and Lee Sin who were strong before the change). I look at Runaan's Hurricane and see love for previously maligned ranged champs with heavy onhit aspects, like Varus and Twitch. I also see changes to Wit's End, Phage, Trinity Force, and Ionic Spark (to Statikk Shiv, basically) as penalizing previously strong champs like Jax, Irelia, Ezreal, and Shen.
So does Riot really make item changes to strengthen that which is already strong? Does your thesis hold in the face of recent evidence?
Also, something worth noting is that Runaan's Hurricane is actually a very gimmicky item. It sounds good on paper, but when you think about it there are several issues you run into.
1) How often are people going to actually be grouped close enough to where you can get the maximum effect of this item?
2) In a 1v1 situation you are paying for attack speed alone, I'd rather have a Phantom Dancer or Zephyr.
3) Blood Thirster, Zerker's, Phantom Dancer, and Last Whisper are core on a lot of AD carries. Do you sacrifice Guardian Angel/Mercurial Scimitar or Infinity Edge to acquire this item?
Also, how was the change of Ionic Spark to Statik Shiv a nerf to Ezreal exactly? Even then, that item is extremely gimmicky too. I wouldn't buy it unless I was a split-pushing ADC. The only one that comes to mind is Sivir and she already does a good job at split pushing with PD and BT.
The way I see it: more itemization was added for characters that already had a lot of strengths to begin with, whereas the characters that were weak to begin with weren't so lucky.
Here is an example of what I mean when I say that the current itemization only serves to make the strong even stronger. Seraph's Embrace is really strong on champions like Singed, Karthus, Swain, Anivia, etc... largely due to the fact that it is so easy to get mana with the item now. This made characters that were already strong even stronger. Don't even get me started on Muramana on Ryze.
So to answer your question, yes, my thesis does hold water in the face of recent evidence as the increase in champion diversity has been largely marginal whereas the increase in item diversity has shot up.