1) Steppers are designed to run hot. There's no harm in them in them being hot. Not sure if these "hybrid" motors actually run cooler, because they are still steppers.
2) True, but I can't hear my steppers when the machine is cutting.
3) Your machine will require a certain amount of torque to achieve a certain performance goal. That torque doesn't change with a stepper or servo. To utilize a smaller servo, it must run at much higher rpm, and you need to add appropriate gear reduction.
4) My stepper driven gantry machine has never racked in the 7 years it's been running.
In my opinion, steppers with encoders is a fix to a problem with the user, not the machine.
A properly setup and maintained stepper driven machine should not lose steps, ever.
People lose steps mainly for one of two reasons.
1) They want the machine to go faster than their steppers are capable of. Adding "servo steppers" to this situation will result in drives that fault, rather than lose steps. Your losing steps from a lack of power. Adding the same size motors will still leave you with an underpowered machine.
2) There's a problem somewhere with the electronics. THis could be anything from the PC or mach3 not outputting signals properly, all the way to drives that are prone to resonance.
THese "hybrid" systems are not exactly cheap. For not much more money, you can get a real servo system which should provide much better performance, if the servos are sized properly.