Thanks for the replies, all.
The reason for asking was that BMI are now offering lifetime Gold cards after 10 years. As has been pointed out, "lifetime" means the shorter of the lifetime of the cardholder or the airline! And in BMI's case, one not unlikely outcome is that it will one day become Lufthansa-GrossBritannien.
It looks 50:50 at best whether LH would be likely to honour this BMI offer, should they end up taking over the company and the FF scheme.