The below is my opinion:
If you get the right components and amp combination, the factory radio can and will do just fine.
I have the factory Boston Audio system option.
I have only added a sub/amp to mine so far, but I did experiment with replacing the factory amplifier (that dirty seven channel mutant of a thing). I originally replaced the Boston amp with a small 4-channel Alpine, but it really lacked depth and felt a little empty to me.
Later, just for science, I replaced the small Alpine with a monster Alpine 3566 (6 channel) amp. The 3566 sounded incredible. I hooked the 6x9 woofers in the rear to it's sub channels, and then I connected the rear highs and front speakers (not including the center channel) to the other four outputs. It sounded really, really good. I actually considered keeping the factory speakers after hearing what they could do with a quality amp pushing them.
I only went back to the factory amp because I'm having trouble deciding where to mount that massive Alpine.
As far as "doing it right" requiring the replacement of the factory radio, I just don't believe that. I believe that the factory radio has the ability to do the job just fine, but I highly recommend replacing that factory amp (if your car has the Boston option).
Hopefully, I haven't overwhelmed you