Licenses and revocability, in a paragraph or less. (for the lay-man)

[freedomfromruin]

As has been stated in easily accessible terms elsewhere:
"Most courts hold that simple, non-exclusive licenses with unspecified 
durations that are silent on revocability are revocable at will. This 
means that the licensor may terminate the license at any time, with or 
without cause." +

Version 2 of the GPL specifies no duration, nor does it declare that it 
is non-revocable by the grantor.

(Also note: A perpetual license may violate the rule against 
perpetuities in various jurisdictions where it is applied not only to 
real property but additionally to personal property (and the like), 
which is why the GPL-3's term of duration is set as the duration of 
copyright on the program (and not "forever"))

+[https://www.sidley.com/en/insights/newsupdates/2013/02/the-terms-revocable-and-irrevocable-in-license-agreements-tips-and-pitfalls]