The best argument for favorable LTCG taxes is that naive taxation of long-term capital gains as current-year income in a progressive tax system overtaxes income that takes an extended period to generate and isn't repeatable, but allowances for advance recognition and/or deferral of recognition of income for tax purposes solve that and are also usable for non-capital income that is concentrated over a shorter time than the work to generate it (as is often the case for creative work.)

If you want to encourage sales of idle assets of a particular class, tax the asset, don't lower the capital gains tax.

The capital gain tax can't be both very high and very low and you seem to have agreed that very high is the wrong incentive.

So what remains?

A low capital gain has the benefit of making it easy to sell so more people will sell. Not perfect, perhaps, but an incentive to keep unused/unwanted properties on the market is better than pushing people to hold on to them forever which a high capital gain tax will do.