A person could write a report or a novel on an iPad, but even with a bluetooth keyboard, text editing functions on the iPad don't even begin to approach what the Mac can do. The same goes for photo, audio and video editing, working with large spreadsheets, complicated research, etc.
In fact, I'm writing on my MacBook Pro right now at Starbucks amid a sea of other MacBooks. Here, Macs outnumber iPads 5 to 1 and there is only one lonely non-Mac laptop. This is a large Starbucks mostly filled will students.
When I write a tip for Basics4Mac, I am moving between many Safari tabs and getting info from several different apps. Once written, the tips go through one or more editing passes. This is simply not work I could efficiently do on an iPad or iPhone.
If your computer use is mostly Facebook, crosswords, email, checking the weather, reading or quick lookups ("Siri, who was the 33rd president of the United States?") an iPhone, iPad or other tablet or smartphone will probably do you fine. They are cheaper than a Mac and easier to use in bed.
There is still plenty of room in the world for Macs. For those of us who do more complicated work, the Mac is still the best tool for the job.