No, everything doesn't have to be highpoly. High poly is just for baking. So if you want to capture detail and silhouette edges to a normal map you'll need a highpoly. Unless of course you just create a chamfered lowpoly and add the normal details in something like NDO or Substance.
I recently heard David Lesperance(well known environment artist) say he doesn't ever create a highpoly and just models the lowpoly with chamfering.
It depends a lot of things: the target platform, the purpose of the asset, the importance of the asset, the game style, the workflow within a team. If you're just learning this stuff then create the high poly purely for the experience and practice, if nothing else.
You can do a lot without hipolies and instead use material atlases and custom vertex normals. Take a look at Alien: Isolation or some recent Assassins Creed games for instance.
I generally don't make high-poly models unless absolutely required. The details I want can be added with NDO or modeled in directly. Aligning vert normals can go a long way toward improving shading without a bake as well.
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I recently heard David Lesperance(well known environment artist) say he doesn't ever create a highpoly and just models the lowpoly with chamfering.
It depends a lot of things: the target platform, the purpose of the asset, the importance of the asset, the game style, the workflow within a team. If you're just learning this stuff then create the high poly purely for the experience and practice, if nothing else.