I'm sorry, dmargineau, but much of what you say in your post is not true.
First, a DC motor develops constant torque (not power) down to zero speed. An AC induction motor with a good sensorless or flux vector drive can do the same thing. The shaft does not need to be turning to develop this torque. The stator field will be turning only enough to generate the required slip and torque but the shaft can be stationary.
Second, there is, therefore, no need to upsize an AC motor when comparing to a DC motor unless the DC motor-drive system is sized for more than 200% shortterm overload torque. And, those systems are rare.
Third, it is far easier to overspeed (or field-weaken) an AC motor than a DC motor so, if the high torque conditions occur only at start or low speeds and the motor's power train ratio can be changed (as in belt coupled loads), then by increasing the power train reduction ratio and compensating with overspeed, you can go above 200% shortterm torque (up to about 300%) without upsizing the motor.