No-one has previously (to the best of my knowledge) reported such an occurrence. Have you personally experienced such an occurrence with regard to your own account? If this is true, then the system is fairer than I had thought. But it's strange that it's taken so long since the implementation for such a report to be made (or if it has, I missed it, and I suspect a lot of others did, too).

Perhaps it's just down to the fact that there are probably so few people (particularly on a forum like this!) who earn miles into FB by means other than flying but go for so long without ever taking a mileage-earning flight.

While I concede that the statement you highlighted in bold above does indeed appear to indicate that each Award Mile has a lifetime of 20 months, I think the later statement is a bit less positive:

The Company reserves the right to cancel the Award Miles after a period of 20 months with no extending activity registered on the Member’s account.

After 20 months of no level miles, the company reserves the right to cancel "the" Award Miles. No differentiation is made here as to the "age" of the Award Miles. They're referred to in their totality.