While American Express imposes a once per lifetime rule for bonuses, there's many other ways to earn Membership Rewards, so earning the bonus on the Platinum Card doesn't limit you from other earning opportunities like the Prestige does. (uponarriving.com)
Calculating how much you need with the rule of 300 can help you quickly assess how to achieve financial freedom in your lifetime (andrewstotz.com)
Despite some confusion on this point, Amex's once - per - lifetime rule applies to their signup bonuses, not their actual cards. (travelcodex.com)