Welcome, Noelle_Carter!
If you take a look at what Metro was replying to, it was a question from Audren about whether previous heirs had to live the entire time, or were allowed to die. In that respect, the reply is then, "the previous heirs are allowed to die. In fact they will most likely be your main source of large tombstones."
This is still a workbench, so that requirement may change, but for now, founders and heirs will be the source of the obelisk tombstones.