On the contrary, Apple is the only one who has this market figured out.
Ever see anyone at a coffee shop using a Surface Pro in tablet mode? I haven’t. That’s because the second you sit down to complete tasks for more than a minute or two you’ll want a keyboard at least if not a mouse, too.
In my view that eliminates productivity as a legitimate use for a tablet. There’s no way I’d prefer touching my screen to the kind of keyboard and trackpad you get on a MacBook Pro.
Next up, content consumption. As you mentioned, your phone covers this use case. I watch videos on the bus nearly every day and never desired a tablet. If you need that bigger screen, you’ve either already got a computer because you do productive work, or you buy a cheap tablet because you don’t need all the extra stuff a computer has, or you’ve got a bigger phone. Why would I watch content on a tablet when my laptop or TV can do that? A tablet doesn’t even stand itself up on my lap. I think only kids tolerate consuming content this way.
Finally, there’s the creative market. Apple has this nailed down with the iPad Pro. The only other reason to need a tablet over a phone or laptop is to do digital art. These are probably the only folks who need to buy two devices, but they’ve needed to buy expensive computer peripherals for ages.
So I think what you’re asking for is for someone to find the really tiny market who wants their tablet to do full computer stuff but doesn’t prefer a proper laptop computer, and then make special software modifications just for them like Microsoft did.