To be extra clear and restate it again, I'm arguing that a well-designed federal inheritance tax policy would include provisions to raise the exempt threshold for small business ownership stakes to something like 30-50MM when that ownership is the primary source of income. There's 40-something million small businesses in the country, so it's not as uncommon as scenario as you seem to think for an owner to pass down their share.

Here's some 2013 data on small businesses: https://www.sba.gov/sites/default/files/advocacy/United_States.pdf

28.7k small businesses in total, of which 23k are nonemployer firms which are defined as the smallest kind, many of which are solo ventures that are principal sources of income. So considering that's the extreme undershot number since the parterships/incorporateds would have several additional owners also reliant for primary income, I think this is a vital provision for any rationally constructed inheritance tax policy.