I suggest, however, that the old three-foot-rule may very well be outdated and obsolete. Why?
Because modeling audiences have changed.
Back in the day the three-foot-rule became a thing, most people had but a single audience for their model railroad. A relatively local group of people that could come see it in person. This grew a bit if you were near a convention and chose to open your layout to tours, but generally speaking, the majority of the audience for your modeling work were those that could visit your basement.
A secondary audience was to be found at meets (particularly in contests) where you could bring a diorama, display, or locomotives and rolling stock. The best of your work would be seen by a larger audience, although outside of the context of the layout. As such, the layout might be held to a lower standard than the contest models.
For a smaller group of modelers, the audience was expanded by being featured in the modeling press. A magazine article on a layout moved the basement into homes across the world. So at least portions of the layout would be made more "contest ready" for the carefully staged and photographed segments of the layout. The layout may not even be finished, but that didn't matter since you could frame the photos to feature the finished portions.
So what's changed?
The internet and digital photography.
Nowadays, the vast majority of the people likely to see your modeling efforts will likely never see it in person. And like the magazine photographs, everything in the pictures is "scale size." Things that looked OK at three-feet might look unfinished, dramatically oversized, or out of place. This is where the three-foot-rule begins to fall apart.
One reason I've already mentioned - everything is blown up to 1:1 scale in a photo. But the second reason might be the bigger factor. Folks looking at it can study it like they think Waldo is hiding somewhere in the layout. Each photo captures all of the details, and practically invites the viewer to look at the smallest ones.
This is, by far, the largest potential audience for your work. It doesn't mean that everything has to be super realistic, or perfectly accurate. But I do think that when building a model and a layout, it is worth considering what the rest of the world will see of your work, and how you want to present it.