I don't think Melkor was paying much attention to the what the music was prophecying. Also, not being able to concieve of Earendel, what else could Morgoth have feared from Turgon, other than military force? Why else would he have used such force. I think Morgoth saw Gondolin as a possible threat, but only a vague one. He is just as likely to have used devastating force simply to show off.
Morgoth's behaviour just doesn't make sense to me unless he really is laid back about the whole thing, or at least feels he has plenty of time to take over ME.
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