That's what they're doing, although not with the exact tiers you mentioned.
As for the $30/mo unlimited, I believe part of the reason why AT&T removed the unlimited option in the first place is because a disproportional amount of bandwidth is spent by people who jailbroke their iPhone and use 100+GB/month using it as a 3G modem.
As if anyone can actually get a good enough signal to pull 100GB off the AT&T network. You have to be masochistic to use a AT&T 3G exclusively.
Also, you don't need to jailbreak to tether. I forget the details, but basically you go to a web address and install a new carrier profile and thats it.
If AT&T want to be the big boys on the block they have to invest in their network, end of story. The reason they got rid of the unlimited option is because they want to make all the money and not do a thing to improve their network.
As for the $30/mo unlimited, I believe part of the reason why AT&T removed the unlimited option in the first place is because a disproportional amount of bandwidth is spent by people who jailbroke their iPhone and use 100+GB/month using it as a 3G modem.