AT&T ATT Wireless Forced Changing of Rate plan , Internet

Good luck finding a competing carrier that will offer you a rate plan like that. Any Carrier with decent coverage certainly wont. You could switch to metro or boost and pay that much then have very poor coverage/ signal. 
Time to get with the times. If I had a cable provider from 1979 and never changed my plan I would expect my bill to be the exact same today. Inflation happens

It does not sound like you are under the contract anymore, so you wouldn't incurr any early-termination fees.  If that is the case why wait until May 31st?  Why not leave now?

However, while you seem to have left out some details, and right now it does not seem like a RipOff. 

Because if you have kept this "plan" through the ATT-Cingular-ATT period, it appears that your rate has only increased just under $7 in about 5-6 years as all of that started in about 2005-2006.  Although I don't know where the $52/month comes in.  It is hard to tell if they were giving you a $52/month plan for $29.99 a month, or the $52/month was what it was costing you.  If it is the former that hardly sounds like a RipOff.  If it is the later it sounds like you have a lot of overage minutes, in this case you probably could have gotten a more expensive base plan but because it includes more minutes the net cost would have been less per month.

You are also talking "mmode" and that is very old technology, I am not even sure if they use "mmode" anymore.  So you probably have a very old phone.  It is unlikely that any phone you get now would be worse than the one you have.

But on what you have written it looks like you would be better off with a Pay-As-You-Go phone.  That way you have no contracts and you can pay the amount you want.