AA was offering Plats (ie BA Silver) the chance to keep status this year for $750 (c£400), so a DCF valuation would probably be about £5k - or 312,500 miles at BA's 1.6p valuation.

BA is unlikely to ever offer this for miles, however, on the basis that people who fly enough to get the miles should have status anyway. Even AA is only accepting cash, not miles!

If you wanted lifetime lounge access then you could get lifetime Qantas Club membership, although the chance of the BA / Qantas link remaining the same for the next 75 years is nil.

Giving 'lifetime' status as a reward for loyalty seems a particularly American trait. Only charities and groups like the IOD (where money changes hands) seem to do it here.