Second bad assumption is that valuations would plummet because "everyone" would be trying to liquidate. The current proposals only apply to a small percentage of the population, and to a small percentage of their wealth, so obviously not "everyone" would be liquidating.
The third bad assumption is that this would lead to government takeover. By your own (contrived, alarmist) situation, the government would only be entitled to 2% of a persons holdings.
BTW, the IRS already does require private companies to perform regular valuations to track how much their shares are worth.