History of "X is dead. Long live X"

Typically it's used in the phrase, "Le Roi est mort, vive le Roi!" or "The King is dead, long live the King" This means that the (previous) king is dead and we wish the (new) king a long life. It's also to indicate that there is never a time without a king.

Wikipedia goes into more detail

The examples you list typically mean something like, "[language] was bad, now [language] has been improved, or is being used in a totally new way."

The original phrase is

The King is dead. Long live the King!

According to wikipedia:

The original phrase was translated from the French Le Roi est mort, vive le Roi!, which was first declared upon the coronation of Charles VII following the death of his father Charles VI in 1422.
...
The phrase arose from the law of le mort saisit le vif—that the transfer of sovereignty occurs instantaneously upon the moment of death of the previous monarch. "The King is dead" is the announcement of a monarch who has just died. "Long live the King!" refers to the heir who immediately succeeds to a throne upon the death of the preceding monarch.

In these modern variations, the apparently contradictory phrase is used as an attention-grabbing headline to appeal to the reader's curiosity.

Something the other answers here are missing is the bitter irony of the phrase. The exclamation "Long live the King!" is used to suggest that it would be a horrible tragedy if the King were to die. The implication here is that the King is such a good King that the people want him to live a long life so as to continue such a good rule.

Then the King dies and another King arrives and... nothing changes. The people immediately begin wishing the new King long life which sort of implies that, as much as the tragedy of a King's death is horrible, the old King was just as good as the next is expected to be. "The King is dead. Long live the King!"

Its use with regards to modern technology doesn't really fulfill this irony well, however, and it is more often used to imply that as one use of a technology dies the technology will survive in another form. People often call out that one technology or another is "dead" and so this phrase finds a use keying into the idea that something can "die" and then survive in another way or form. Likewise, some technologies are notoriously stubborn and refuse to go away no matter how much we all want them to fade into the history books. (coughCOBOLcough)

I think I read somewhere that the original phrase was "The King is Dead! Long live the King!"

Kidding. ;)

To me, the Javascript example means is that there no suitable replacement. If a web developer were to say, "Javascript sucks! I will never use it again!," then he would quickly find out that there's no alternative to it, and would basically have to go straight back to Javascript once again. To me, it sort of signifies some sort of roller coaster ride type of relationship -- even if you hate it, you're going to have to learn to like it.

Perhaps an analogy would best explain it: Imagine, if you will, a wife that screamed at her husband, "You're an idiot, and I really can't stand to be around you! This marriage is over!" and when she opened the door to walk out, she saw some guys outside that were drinking, playing with dog crap, pissing in bushes, etc... prompting her to shut the door immediately and change her mind: "Just kidding! Teehee! Love you, babe! You're the best!"