You make it sound like factories don’t actually net them profit LMAO. Even after paying taxes they’re still making money. If they weren’t, it would be a terrible business. Also what do you mean sell them to whom? Other billionaires that still exist even with a wealth tax, non billionaire investors, international investors. If they can’t find someone to sell their shares to, then clearly their shares are overvalued and that’ll take care of some of the problem in itself.
Also shutting down factories won’t affect their wealth, which is actually what’s being taxed. And it won’t matter weather they sell $1 billion of whatever to buy $1 billion of something else. You’re taxing their wealth, not their business or cash in hand. If they’re holding on to $1 billion in one form or another, then they’re holding on to $1 billion that can be taxed.