You're assuming that there will be no buyers for sellers, and...

You're assuming that there will be no buyers for sellers, and you're making the assumption those that are not NG will somehow stop paying taxes.

You are also ignoring that those who are NG will be unable to reduce their tax burden on income not related to the NG property.

My assumption is that net tax takings will improve, though I'm not sure by how much.

If someone wants to sell, there will always be a buyer, so long as they meet the market.

Those who are not NG will not be affected by the removal of NGing.

Those who used NG will either pay the tax they were previously avoiding, or they will invest their money into other ventures.

Those investors who leave the property market may actually invest in business ventures which would be more beneficial economically for Australia.