A Tolkien Theory: Why Couldn’t Morgoth and Sauron Change Bodily Form?

A recent video by TheRedBook on Sauron’s death (https://www.youtube.com/watch?v=NM5hrpjsoM8&pp=ygUbc2F1cm9uIGRlYXRoIG9mIGEgZGFyayBsb3Jk) reminded me of some pertinent passages in Tolkien’s writings that may help build a more solid theory for why Morgoth and Sauron, after a certain point in their trajectory toward evil, could no longer change their bodily forms, which is why Sauron in The Lord of the Rings had only nine fingers (as I discussed here: https://www.youtube.com/watch?v=L1XfiLndomE&pp=ygUgd2h5IHNhdXJvbiBvbmx5IGhhZCBuaW5lIGZpbmdlcnM%3D)

The GirlNextGondor video on redemption I referred to is here: https://www.youtube.com/watch?v=G5-Xc21Kvsc&pp=ygUZZ2lybG5leHRnb25kb3IgcmVkZW1wdGlvbg%3D%3D

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