Modern fat bikes will be more efficient than walking in the scrub desert where I live, in grasslands, in not too dense woodlands, on any kind of open dirt/sand.
I've done 100 mile MTB races, and while it's not as efficient as riding a flat smooth road on a road bike, it's probably still more efficient than walking.
Obviously at some point there's a line, where you can't ride a bike, but for most roads and trails, the bike is going to win.
I think that the comment I was responding to was about scenarios where are no human-created pathways, trails or roads. As I mentioned in some of these, modern fat (tire) bikes are still great; in others, it is true that walking would be the best choice (unless you're at the level of, say, Danny MacAskill).
But yeah, in some natural environments, with some kinds of bikes, you can beat walking. The Burning Man festival is a great example, but it is also a terrible place to live.
That's why in most cases, without smooth roads, bikes are not practical.
I think it's fair to point out that you should be referencing road bikes then when you say bikes are not practical. Advancements in tubeless technology, suspension, MTB groupsets with dinner plate low gears, derailleur clutches, and hubs/rims designed to take a beating while supporting wide tires can definitely make a bike more efficient than walking on most terrain.
- Fat bikes are more efficient than snowshoeing or breaking trail on XC skis.
- CX bikes are more efficient than walking in mud.
- Fatter tire gravel bikes are more efficient than hiking through sand.
- A bike with a 51T cog and 28T ring will be more efficient than hiking up steep grades until balance at low speed becomes an issue.
They don't require a flat and smooth surface, but do benefit from a road of some sort. How do you make a road? You ride on it over and over again. Ever seen a sheep track? Is that not natural?
A smooth and flat road, while not necessary, is better and does make things a lot more efficient. The same is true for any wheeled vehicle but cyclists appreciate it a lot more than motorists.
...On flat smooth surfaces. On any natural environment or terrain they are nowhere near as efficient as walking.