They'd probably try to rely on legitimate interest rather than consent, but it would depend to a large extent on how personalised and automated the messages were.
Good luck with that. Most legitimate interests require the reason to be defensive or a relationship to exist where there's an expectation they would need to process your data. I am legitimately interested in spamming you is not really a defense.
To be clear, I wasn't defending it - I was just saying that they'd probably try to claim that rather than consent. I agree that they'd likely fail if it actually came to a court case.