Yes the ipad3 is just as good today as it was yesterday. BUT, it's is worth less.
The key here is depreciation, I assume that an Apple product in this category lasts around a year before it's replaced by a newer product. It's not about having the latest is that while it's the latest it still has a higher resale value.
Releasing something in 7 months rather that 12, has suddenly depreciated the iPad 3 much faster than normal.
So there is a genuine quantifiable reason for iPad 3 owners to be unhappy.
Apple should consider an upgrade program in which you can pay a reasonable amount to upgrade to the new iPad.
Your 3rd gen iPad is like COBOL and the lightning connector is like Y2K. There's going to be a demand for the 3rd generation model precisely because it's not the latest and greatest. People with drawers full of 30-Pin cables are going to want to use those cables as long as they can. So long as you don't try to scam them into paying the full price, you can get a good trade-in value for your vintage 3rd gen iPad.
It's even better if YOU are the one with the drawer full of 30-pin cables. Since Apple probably won't refresh again for another year, you can keep your iPad 3rd gen for 18 months instead of 12 before selling it. You might even get a few bucks more for all those cables you won't be needing any longer.