Well, who said that the Valar could forsee the future? They were not GODS in the sense of omnipotence and omnipresense and everything. They were just wise and immortal beings.
And as for Eru, he would not care since
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none can change the music in my despite, and Melkor shall find that all at last only redoundth to my glory
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Maybe Eru moved the heart of Boromir to go himself anyway, for that matter.