The purpose of the old English rule was to prevent land ownership from being permanently tied up within a family. The impact is that transfer taxes can be avoided for the rest of time, not just 100 years. With compound growth, the savings can be incredible. The states that now allow Dynasty trusts are Alaska, Delaware, Idaho, Illinois, South Dakota and Wisconsin. Some people create long-term trusts to ensure that wealth is forever available to their descendants for medical and educational needs.
Some people create long-term trusts in order to control their estate from the grave.
Perhaps the greatest motivator is to prevent Uncle Sam from taking more than half of the family's wealth at each generation. There is a gift or estate tax cost to create the trust. But whatever stays in the trust, including income and appreciation, avoids federal transfer taxes when future generations die. However, when the traditional trust is eventually forced to terminate at an arbitrary date without regard to the needs, circumstances or capabilities of the recipient, the distribution may be subject to transfer taxes either at the time of the distribution or when the recipient dies. On the other hand, if the trust was a Dynasty Trust, there would be no forced termination and any property that is not distributed can avoid transfer taxes forever.