Any device is a compromise.
Perhaps a device with a 7 inch screen and a small bezel that has detachable keyboard and operates through a Bluetooth headset for the phone function.
Or maybe a larger tablet that has your cellular/data connection, combined with a dedicated Bluetooth telephone handset.
It is just silly to say things can't get any better. In a couple of years somebody is going to come along with something that we have not foreseen, and will wonder how we ever lived without it.
Which is why there will never be one device for all purposes. The compromises become too great. You wouldn't buy a combination hammer/screwdriver/wrench, or a combination house/car/wallet. I can watch movies on my phone, tablet or computer, but I have a 50" TV for that.
Perhaps a device with a 7 inch screen and a small bezel that has detachable keyboard and operates through a Bluetooth headset for the phone function.
What you're describing is a tablet. My iPad Mini does that, but it's bigger than 7".
It is just silly to say things can't get any better. In a couple of years somebody is going to come along with something that we have not foreseen, and will wonder how we ever lived without it.
I'm not arguing that things can't get any better -- it's almost inevitable that they will. I'm arguing that converging all devices into one wouldn't be "better."
I have a desktop machine with three monitors, which allows me to have vast amounts of information at a glance. I don't have to do a lot of flipping between windows. I have a laptop, but haven't used it in quite a while because the iPad does everything I need in a portable device. and I have an iPhone that fits in a pocket and has practically become another appendage. Any one of those devices can perform the tasks of the others, but not well.
I have access to a subcompact car, pickup truck, minivan and motor home. I'm not holding my breath for someone to come up with a vehicle that can fit in a small parking space, sleep eight, have its own bathroom, haul a half-ton of gravel, and get 30 miles to the gallon. Physics is a harsh mistress.