There are reasons why iPad does not replace the Mac. It’s not usability. It’s capability. Wil has an opinion on iPad’s woes:
After a few hundred million iPads were sold and Apple’s customers decided to keep them forever instead of getting new models every couple of years, the bandwagon hit a brick wall. iPad sales took a nosedive because Apple did little to change the form factor or give customers more reasons to buy upgraded models. An iPad is a tablet. That’s all it was. That’s all it is. A handheld touchscreen device for the masses.
What’s the problem now?
Input. It’s all about input. iPad, like iPhone, is a touchscreen device and that’s exactly how most of us use iPads; regardless of how old they are. They are, for the most part, a content consumption device that can double up and do more, but usually falls short of the heavy lifting capability you get with a Mac.