HSA usage is not defined by eligibility to fund an HSA. While you can no longer contribute to your HSA if you don't have an HSA-qualified health insurance plan, you may continue investing and/or paying for qualified medical expenses with your HSA funds. HSA eligibility does not hinder this HSA benefit.
An HSA is truly a lifetime savings account that is always there for health expenses.
If you no longer have an HSA-qualified health insurance plan, you can't make contributions to your HSA.