It is happening!
But no one "supposed" for DVDs to disappear. It's a prediction based on market forces, not a pronouncement by some dictator.
DVDs are still sold because people buy them. And they still buy them because they are cheaper than Blu Ray disks and there is a large installed base of DVD players and non-HD TVs.
In 5-10 years or so when most of the old non-HD TVs have died, then you can expect for pre-recorded DVDs to slowly become extinct. Eventually it will no longer make economic sense to manufacture them.
It's no different than pre-recorded VCR tapes being made long after DVDs became a main stream product.