If UA has 38 million MP members (which it does) and only about 40,000 top tier flyers (which it does) lets guess that there are MILLIONS of semi-dormant accounts with a nominal amount of miles in them.
Say 5000 miles average in a million accounts. (Remeber a r/t SYD-LAX is 15,000 miles and many first time flyers rack up that and never fly again. LAX-LHR is 11,000 r/t etc.) That is 5 BILLION miles. Do you really think UA is worried about that???!!! No way. Firstly, there is no earthly use for 5000 points. Secondly, if that person ever took another long trip it is likely they wil think UA first, not last, to "add to my account".
It is called "Golden Handcuffs" in marketing language.
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~ Glen ~