New investors can buy Steem and hodl it without powering up, and withdraw it whenever they want. It's up to them whether to power up or not.
How would this plan stop current investors from powering down, creating new accounts, and getting the delegated SP?
Your are right, powering up or holding as liquid are the options investors have. Powering up might not be attractive for some because of 3 month lock of assets. Holding as liquid may not be attractive for some because there are many other options.
Third option I suggeated, I believe would attract many investors who already hold crypto assets.
Steemit Inc has enough SP to cover all new or old users. I doubt many whales would take advantage of it, as they would lose an ability to delegate themselves.
Once investors are in and get to know the Steem blockchain I believe they would choose to power up themselves.