Infinitely linear torque with current in motor?
If you push enough current into the coils of a permanent magnet motor, then eventually you will demagnetise the field magnets. This is NOT GOOD for the motor. However, this current is usually fairly generously above the maximum normal running current, which is a thermal limit, and can be exceeded for short periods. But how much above?

With brushed DC motors, it's quite easy to get a figure for the safe maximum current, and that's the stall current when the rated voltage is applied.

With brushless DC, you'll need to dig through the specifications. A reputable manufacturer will tell you.