The difference is that IHG has to convince owners and managers, essentially suppliers, to give free stuff to people.

IHG has enough of a problem convincing them to do this, even where the people are recognised as frequent customers. If those people no longer have an incentive to stay at their hotels, I can guess what the owners' response would be.

Clearly we would all LOVE life-time membership. It would free many of us to enjoy occasional stays at IHG hotels while maintaining high status with other brands. It's facile to manufacture positives for all parties in life membership, but those scenarios are unlikely to map reality.