You said: "Before that, you in practice had been limited to synchronous permanent-magnet DC motors." Series-wound DC motors don't have permanent magnets at all

You said: "Before that, you in practice had been limited to synchronous permanent-magnet DC motors." Series-wound DC motors don't have permanent magnets at all -- rather, they have two sets of windings (connected in series) that push against each other, so there would be no concerns about rare earths. I don't know what a "synchronous" DC motor is, unless you meant to say "synchronous AC and permanent magnet DC."