You are trying to shove the future into a present-day sized box.
If things were working correctly, there wouldn't be any need to take your phone out of your pocket to have it drive the laptop.
And the laptop needn't be useless if the phone isn't around, it should just be able to advertise its screen and keyboard to the phone, so that a user that wants to push text at the phone can use the laptop to do it (without messing around with much of anything).
It really depends who far into the future we are thinking. If we have the technology to reliably stream high resolution video (of the desktop/UI) through the air with no noticeable latency at 120fps then all bets are basically off.
If things were working correctly, there wouldn't be any need to take your phone out of your pocket to have it drive the laptop.
And the laptop needn't be useless if the phone isn't around, it should just be able to advertise its screen and keyboard to the phone, so that a user that wants to push text at the phone can use the laptop to do it (without messing around with much of anything).