Of course you won't forfeit any miles. Two reasons: 1) you already earned them from the credit card spend, as long as you pay the bill, and 2) some of those miles are for flights flown, having nothing to do with the credit card.

The companion fare is the most valuable aspect of the card, so if you aren't going to use it, then sure, cancel. Just remember if you change your mind in the future and want to reapply it will be under the new rules (e.g. the $6,000 spend requirement).

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