Why all those heat sinks? Power electronics are getting very good these days with low RDS-on. Have stepper drivers not kept up?

Sadly not really.

I think we're only a few years away from BLDC servo motors taking over from steppers in 3d printers.

Ideally control algorithms for them would go into the MCU so there is proper force feedback too - ie. The system will know that there is an extruder clog by the increased extrusion force, or even set print speeds to be 'the fastest you can follow this path' rather than a fixed number of mm/sec. Ie. If the bearings get a little stiff it'll go slower rather than skipping a step.

There are some patents on sensorless servo control expiring which should cut the price of this stuff almost in half since the position sensor is one of the most expensive bits.

Power supplies are one of the more expensive parts of a 3d printer, and by having BLDC motors which can do regenerative braking, that same energy can be reused in the head and bed heaters, which should allow significantly smaller power supplies too - again with significant software complexity to make sure the bed heater primarily heats when the head is decelerating and stops heating whilst accelerating to not exceed the power budget.