Originally Posted by apcd2

Not all but a very large number (maybe even the majority) of tier points are earnt by flyers on corporate deals. These people have no choice but to fly BA. As a result I suspect BA are very happy to see their extra tier points disappear at the end of their membership years. Someone earning 450 tier points a year will never progress beyond bronze under the current rules. Allow tier points to roll over and they'd be silver half the time costing BA more money but bringing in no extra revenue.

There's always the carrot of lifetime tier points and gold membership to strive for.