Oops, you're quite correct; I should have said "over a decade". As for the dollars, they really aren't any more. The only way to get them (Presidential or Sacajawea) is directly from the Mint at a healthy premium, thirty five dollars for a twenty dollar roll of coins or in a mint set at an even healthier premium. Congress tried to force the Presidential dollars in particular down our throats by mandating high mintages but all that happened was they just piled up in banks. I don't know why the dollar coin is so unpopular, I prefer using them to paper myself.
As for the half, after Kennedy's assassination Congress authorized a new design for the half dollar. Typically durint the first year of issue of a new series there is a lot of hoarding of the new coins. There was more hoarding than usual in the case of the new half because Kennedy was a popular president. More importantly, the following year silver was removed from our coinage and replaced with copper-nickel "clad" coins. All except the half dollar, which instead had its silver content reduced from 90% to 40%. Silver coinage rapidly disappeared from circulation, of course. Dimes and quarters were replaced by their clad variants, but people continued to hoard the halves.
Cash register drawers had five bins for coins, one for each denomination. When the half dollar disappeared from circulation stores started using the now-unused half dollar bin for other things. People just generally got out of the habit of using halves altogether so by the time a fully-clad version was issued in 1971, nobody really cared.
There's more to the story than that, notably the fact that the half dollar is inconveniently larger than the equivalent two quarters, but that's part of the reason half dollars are effectively no longer in circulation in the US.