Even if the owner of the subscription leaves the company, tenant admin purchases (typically known as organizational purchases) continue to work as-is. If the owner was an admin, any license admin (License Admin, User Admin) can continue to manage the licenses. The subscriptions can be managed by any Billing Admin with access to the billing account used for purchase.
If the owner wasn't an admin, the license will continue to work as long as the subscription is active. However, the subscription can't be managed by anyone else.