As of November 1st, you won't be able to buy an iPad 4 either, but can still get an iPad 2.
The iPad 4 replaced the iPad 3 last October. Apple still has refurbished iPad 3s for warrantee replacement.
And the iPad 3 is an older version of the iPad 4, not a newer version of the iPad 2.
Just because they stop selling an item does not inherently mean they no longer support it. They will have plenty of service units available for exchange under warranty or out of warrnty. Heck, you can still do an out of warranty exchange on an original iPad or iPhone and will be able to until eventually, at some point in time, factory refurbished service units start to run out.
And the iPad 3 and iPad 4 are fully iOS 7 compatible so will get iOS 7 updates and security and bug fixes as they come out.
Don't worry, many people will still be using their iPad 3's and 4's even after the iPad Air II (or whatever comes out next) is already in the stores.