If they become worth zero do you not have a capital loss to deduct from future income? So you would gradually recover the tax paid when you got the now worthless shares in the form of future taxes you don’t have to pay.
You would have a capital loss to carry forward, and this is part of why the current rules aren’t great for people who aren’t already wealthy. If the tax bill is a big chunk of your net worth it makes it hard to take advantage of what might be a great opportunity.