But do we have to keep the card? There is an annual fee. If I shop through the skymiles shopping will that keep a regular milage account active if there is no linked card?
You can ditch the credit card - your mileage account is
linked to it, but is nevertheless a totally separate entity. Once detached, the mileage account will need activity once every 18 months.
You can shop from UA's portal, as has been mentioned, or redeem magazine subscriptions for a couple of miles. Or, if you get the Chase Sapphire (or know someone who has one), you can transfer 1,000 points (instant) just before the 18 months are up.