You write it as a hypothetical, but that's actually how SpaceX landed their first first booster. It was Falcon 9 Flight 20, which deployed 11 Orbcomm satellites.
Infographic of the trajectory: https://i.imgur.com/D9BdO86.png
The astronauts considered it extremely risky to say the least. NASA actually wanted to try it on STS-1 (a manned flight on a system that had never been tested before) to see if it worked. In the words of the mission commander: "Let’s not practice Russian roulette, because you may have a loaded gun there." Which carries a lot of weight from a man who was willing to risk a lunar landing.
http://www.tested.com/science/space/460233-space-shuttles-co...
https://en.wikipedia.org/wiki/Space_Shuttle_abort_modes#Retu...
Previous HN discussion: https://news.ycombinator.com/item?id=8447479
If the first stage were to "fly" all the way back to land after separating from the second stage, that would require some combination of a heavier/stronger rocket to hold more fuel, and a reduced payload capacity to orbit.
Blue Origin only does short, up-and-down flights -- they are not trying to achieve orbit so there is no need for horizontal travel. They can simply shut off the rocket and fall back to the launch site.