At work, I’m an engineer and people manager and I find that for many of my common tasks like presenting, taking notes, emails etc. the iPad is fine and portable which is a plus. I do have a 16” MacBook which does get a good bit of use but only for things the iPad doesn’t do well, like SSH and text editing, but if it did I’d sooner take the iPad with me.
Just depends what workflow you as an individual use, I’ve built a career out of making web UIs for common tasks so them working on an iPad for my use case is not surprising to me, I’d suggest looking at what you ‘actually’ do to see if you really always need to carry a full laptop.
RE videos - yep no portable will ever be as good as my home theatre but I can’t watch that on a train or in a hotel, I can with my iPad and it doesn’t get anywhere near as warm as my slightly larger laptop does on a pillow. I’d argue its far more competitive than you’ve suggested.
It's also a great primary computer for users that don't need or want the overhead of a PC, ideal for people like my mother who just want to check email and browse social media.
I do agree that for many users an iPad makes no sense at all, especially if you have a variety of other computing devices that are better suited for your specific needs A jack of all trades, master of none situation.
I can imagine this works out in most demonstration cases.
10" tablets were close but not quite there. I always found myself doing more pinch zooming than I liked.
So I guess outside of artists or those who edit photos, there really isn't much of a market that isn't better served with a laptop or a Phone.