Yeah and at that time almost no civilians owned them. Also none already in circulation were illegal. It was illegal to sell them, not possess them. Now there are 30-50 million in circulation. If they were to ban anything ( which again, I doubt) they would ban the sale and importation of them. A nationwide registration scheme for those in circulation never gets passed in congress and certainly never gets by the SCOTUS ( incidentally, I don't believe Illinois' will either).
The painful thing that could happen if this were somehow to pass would be a ban on sales of parts and internet sales. Something like that is widely anticipated and many people have hoarded parts and lower receivers for years now.
But I digress. The odds of congress passing a ban on ARs is infinitesimal.