China only has a monopoly on willingness to pollute
Rare earths are unfortunately named: they aren't rare. The only reason China currently has a "monopoly" on world production is that mines in other countries have all shut down due to difficulty complying with environmental regulations. The standard rare earth mineral extraction process is quite polluting.
In effect, all other countries have exported their pollution to a willing China in exchange for extracted rare earth minerals. Thus, the "worst" China can do in this trade war is to cause other countries to relax their environmental regulations, whereupon those countries' domestic rare earths reserves will come back online.
Not exactly the "high tech device apocalypse" as the media prefers to portray it.