It seems like there is a new policy as of this week that you can extend your expiration date simply by shopping in the portal, even for something very very inexpensive.
I want to make sure I am not crazy in thinking that they would extend my expiration date for the huge amount of miles I have before I buy something.
The website says
Extend the validity of your Miles or explore ways to spend them
You can extend the validity of your Miles for 2 years when you complete a flight with Air France, KLM or one of our 32 airline partners around the world. Or you can use your Miles to book a reward ticket.
If you're not ready to plan your next journey you can make each day a journey of its own with Flying Blue! Shop at the Flying Blue store
Thanks
https://www.flyingblue.com/en/news/extend-your-flyingblue-miles