That would be impossible to enforce or prove. Bitcoin may not be anonymous but it is pseudonymous. The government could make a case that you own this or that public key hash, but that can be litigated till kingdom come since they have no physical evidence. This is assuming our standards of jurisprudence as we know it would still exist in the future where the government attempts to seize coins. I suppose the government could always torture you. Imagine a world where the government tortures citizens to extract Bitcoin because they are forced into a corner by the abandonment of inflationary currency and they have to resort to actually "earning" money. That seems a bit extreme though (just to collect taxes) even for warlord countries let alone the USA. Which is why it wouldn't happen--there wouldn't be a government at all in that scenario, it would just disappear, and humanity would finally be free as it once was.