You need a lot of rare-earths for the really powerful motors. The rare-earth production is never going to scale to replace all the ICEs. Also, permanent motors are less effective at higher RPMs, so you'd still need a gearbox (though it probably needs just 2 speeds).
Control circuitry without IGBTs kinda sucks. You're stuck with switched serial/parallel circuits, like on locomotives or trams. Regenerative braking would be possible, but very limited because you can't control current/voltage well enough not to mess up the battery.
None of this presents an insurmountable barrier, but it definitely limits the design space.
Wouldn't series wound DC motors have been an option? They're awful for a lot of reasons (low efficiency, brush wear, hard to do regen or even reverse without complex switching mechanisms, etc...) but they're simple and don't use permanent magnets and can be quite powerful.
I don't know when SWDC motors were invented but I would have assumed it was long before the 90's.
Before IGBTs, the main way you controlled a powerful DC motor was by having multiple windings that can be connected in parallel or series configurations, modulating the resulting current. This type of control goes way back to 19-th century, it was used on the first trams.
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."
