Beren and the One That Got Away

[Inziladun]

A while back, there was a thread that questioned whether Fingolfin could have potentially killed Morgoth's physical form when the two of them fought. My thought at the time was that he couldn't have, due to the physical superiority of Morgoth, and possibly the Curse of Mandos. However, some one other than Fingolfin had the opportunity to destroy Morgoth's physical body. Lúthien put Morgoth into a trance when she and Beren entered Angband, and

Suddenly [Morgoth] fell, as a hill sliding in avalanche, and hurled like thunder from his throne lay prone upon the floors of hell. The iron crown rolled echoing from his head. All things were still....Then Beren drew forth the knife Angrist; and from the iron claws that held it he cut a Silmaril.

The Silmarillion Of Beren and Lúthien


Beren got greedy, however.

It came then into Beren's mind that he would....bear out of Angband all three of the jewels of Fëanor.....The knife Angrist snapped, and a shard of the blade flying smote the cheek of Morgoth.

There Beren was, Morgoth helpless in front of him, nothing stopping him from dealing a serious, or even fatal wound to Morgoth with the knife.
Could Beren have killed Morgoth?
If so, I would expect Morgoth could have re-bodied, just as Sauron did. Like Sauron however, it might have taken a long time; time for the Eldar and Edain to gather their strength and devise new strategies against Angband, maybe. At the least it would have changed the history of Beleriand.
Did Beren allow a once-in-a-lifetime chance to slip away?