Airlines don't really use expiration as a way to control this anymore; while they haven't all moved to non-expiring setups, it's usually easy enough to keep current that it won't matter (i.e., in many cases miles expire based on a period of inactivity on the frequent-flyer account, but adding miles through card usage counts as activity).
The way they prevent it being too large a loss is by periodically devaluing, so that the cost in miles/points to purchase a ticket goes up over time.
The way they prevent it being too large a loss is by periodically devaluing, so that the cost in miles/points to purchase a ticket goes up over time.