Thanks so much for that thoughtful response--much appreciated! Let me comment on my situation personally.

The Berkshire Gift to my son would be a tiny fraction of the lifetime exclusion. That's not at all an issue.
MA may be a nightmare state for taxation-- except for 1 tiny exception: it allows unlimited gifting with zero tax consequence

Giving the shares to my granddaughter? That defeats the strategy. You prolong the period to get the step-up by probably decades. Then she needs to die to get the step-up. If I give the shares to my son--when he dies--my grand-daughter gets the shares with step-up

Glad you agree the question of a very substantial forced tax event post Buffett is not inconsequential. I think the odds are higher than you infer because I think Abel will put his imprint on the business in ways that, well, are his--not Warren's. I'm hoping Berkshire considers tax free spin-offs over very large subsidiary sales, but hey, there are worst consequences than having to pay some tax at a future date on a gift from dad, right :)

And I hear you on moving to a tax-free state, but I've seen too many times when a couple does that, one person dies, the other wants to come home to other family members and plan is killed. But great idea if it works --a huge money saver!

OTOH the estate tax on Berkshire holdings IS substantially lower in every estate tax state for people who pass this spring --compared to passing last spring. It's just not my preferred method of lowering the bill.