Just for clarity - miles do not expire. You do not have to worry about losing your balance. After two years without activity they lock your account for security purposes - you just need to contact them to have them unlock your account.
Mileage Plan miles do not expire
For account security purposes, we will continue to lock accounts that have been inactive for more than 2 years. If your account has been locked due to inactivity, call
Guest Care to verify your identity, and they’ll reactivate the account for you. All miles in the account will still be there for you to use.
FAQ from the website:
https://www.alaskaair.com/content/mi...s/faq-benefits