License terms (no matter how explicit) don't override the governing law, and in US law at least, gratuitous licenses are revocable at will; the terms stated cannot change this but may still have some effect under the doctrine of promissory estoppel.

There's also a legal theory that the GPL is, or can be depending on the circumstance of a particulsr case, a contract license rather than a gratuitous license [0], but even in that case the contract offer would be revocable, and the GPL itself only irrevocable with respect to licensees with whom a contract was formed prior to the offer being revoked (because FOSS licenses are sublicensable, that licensee could offer sublicenses to the original work, but might not, e.g., if they chose not to do any of the acts which required such an offer.)

[0] which may not actually save the day, either, since some courts have held that contract licenses are revocable, but that revocation in violation of the terms of the contract makes available damaged for breach of contract rather than compulsory extension of the license.

The GPL appears to be rather clearly a contract license when it governs the give and take inherent in shared open source development within a community. Your points regarding gratuitous licenses seem applicable where publishing has occurred but no further open source development has taken place -- and this seems largely irrelevant to concerns regarding an existing open source community.

I'm not sure why concern would be raised with respect to the original contract offer. It seems entirely irrelevant given that any participant has full rights under the contract terms.