Brakes are an essential part of a car, though I've never used my park/handbrake as primary or auxiliary braking in an emergency stop.
On an older car the test for handbrake includes a balance requirement as it could be used as last resort while moving. On a modern car with split circuit brakes it is much less likely that there wouldn't be enough foot brake to stop. The hand brake is then regarded as a parking brake an the test is just has to hold on slope.
You should try stopping using just the handbrake and see how ineffectual it is. Disc brakes with no assist are not as good as older drum brakes used to be.
Took my old GT4 to a Grasskhana many years ago. Used the Handbrake to help effect cornering on a grass course. Could feel it becoming less and less effective during the day so eventually backed off it.
Afterward I inspected the Handbrake 'pads' and could see they had worn.
They were basically a foam pad that sat inside the wheel rim / drum and pulling the handbrake pushed them outward to contact said rim/drum and hold it still.
If you're using them to stop the car whilst in motion you have really big problems.
(One reason I really object to them being called 'e-brake'. They're a parking brake; to be used when parked, to hold the vehicle immobile once already stationary!)
Locking the 'main' brakes on - which is my understanding of what electronic parking brakes generally do - means you have one less piece of technology to stop with, as it's the same brakes. But again if your brakes fail on all four wheels, you have really big problems!