Good to know. Thank you hhdl.

I ask this question because the phone agent told me I must fly to keep my miles from expiring... I got off my butt and did a little googling. I found the following link:

https://www.flyingblue.us/en/program.../tier-benefits

Excerpt paragraph #12:
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12. All your Miles are valid for life as long as you take an eligible flight on Air France, KLM or any airline partners at least once every 2 years, or if you're a Silver, Gold or Platinum member, or if you make a purchase with an eligible co-branded credit card at least once every 2 years. These "Overall extending activities" extend the validity of all Miles 2 years from the date of the activity. You can also extend the validity of Miles earned with all other partners (airline partners not mentioned above, car rental partners, hotel partners, and experience partners) for 2 years by earning Miles with any of these other partners. These "Partial extending activities" with other partners will extend the validity of Miles earned with any of these other partners by 2 years from the date of the activity. Please note, "Partial extending activities" only extend the validity of partial extending activities that occurred after your last overall extending activity, and do not extend the validity of Miles earned from "Overall extending activities".
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So the difference seems to be - My credit card activities will keep my miles alive forever.