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You should remember that Ecthelion only killed multiple Balrogs in the very early 'Lost Tales' version of the story. At that point, Balrogs were conceived of quite differently from the great demons of fire and darkness that they became around the time of the writing of LotR. If the later 'Fall of Gondolin' had been carried further, it is highly doubtful that Ecthelion would have killed any Balrogs beyond Gothmog.
Of course, you're free to disagree - but one can't simply throw things written years apart together willy-nilly and expect any kind of coherence.
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