There is a primary trustee of the trust and that trustee can add or delete beneficiaries of the trust at any time with a memo to the accountant.
Money from discretionary trusts is taxed either directly when the trust makes a profit from its investments and indirectly when the beneficiaries receive money from the trust, that money will be taxed at the beneficiaries tax rate. Very similar to any company.
Companies can and do use bucket companies to work from and those bucket companies keep very little cash or assets in them.
Discretionary trusts were originally created to pass on a persons estate to those they wished to simply by adding them to the trust as a trustee. That is what they are still used for.
SMSF have their own benefits with tax and operations.
Most people receive tax concessions from the government in some form like family benefits, child minding benefits, public schools, hospitals, low wage earner benefits and the list goes on.