Dylan was & remains one of the biggest borrowers in the history of popular music (even his name was taken from someone else.)
Adapting & reworking other peoples ideas has always had a central role in all art forms.
Especially music.
Whether it is still actually possible to have a wholly original idea is a debate in itself (As is the question of how we would know that we had.)
I think "Hard rain's" debt to "Lord Randall" (to name but one of many examples) is self-evident.
I don't think Dylan has much cause for complaint.