I linked this to a new topic because it has wandered away from the original title.
Please forgive me for the length of my replies. I have a habit of wanting to write essays…
The term “layer” originated in 2D drawing and photo-editing apps, in which it makes sense to imagine that you are looking at a stack of independent sheets that may or may not obscure or interact with the ones “below them”. In that context, it has long been conventional to put related kinds of drawing elements onto the same layer so that you can manage their presentation together: “I want to see how the plumbing relates to the walls, show me just those two layers”. Likewise, 2D apps allow locking a layer so that you can’t accidentally move or draw things to a different, inappropriate one. Each layer is effectively an independent sheet of imaginary paper on which you can draw before stacking them to make a completed document page.
Layout is a 2D presentation and drawing app, so it conforms to this notion of “layer”. The fact that it presents views of 3D SketchUp models does not change this basic aspect of Layout; a 2D presentation of a 3D model is ultimately just another 2D drawing.
However, this notion of “layers” does not make sense in a 3D modeling app such as SketchUp because as you orbit around the model there is no consistent meaning to “in front of” or “on top of”. What’s in front from one direction of view is along side or behind from another. But things can get in the way of each other, in a visibility sense. SketchUp’s layers provide an external place to store a yes/no visibility flag so that by all referring to the same layer an arbitrary collection of geometry can be switched on or off. But that’s all they do; they do not imply that these things are related in any other way or somehow gathered together on some 3D analog of a sheet of paper. So, yes, with hindsight, “layer” was a bad choice of name, particularly since Layout uses it in the conventional 2D sense.
But that does not make SketchUp’s concept a mistake or a design flaw, it just isn’t what 2D CAD people want from “layers”.
It is already possible in SketchUp to use Groups to prevent geometry from interacting with other geometry and for accessing all of some related kind of geometry at the same time. Groups can be locked to prevent editing them, including changing their “layer”. Scenes allow managing the display of different sets of content from different viewpoints. It would seem therefore, that the real question is “how can one organize a model in SketchUp so that all the entities of one sort are mapped to one Layer in Layout, all the ones of another sort are mapped to a different Layout Layer, etc.”? I’m not a Layout expert, so I won’t try to answer that, but no doubt another in this forum can. If it is possible to do, then it is a matter of learning how SketchUp and Layout intended you to accomplish it rather than of demanding that SketchUp replicate the way that 2D apps behave. If not, perhaps this would require a revision to the way Layout imports a SketchUp model.
Regarding the standard advice for how to use SketchUp’s layers, your questions and concerns have been raised many times before. Every time, someone comes forward to explain why the ability to make something other than layer0 “Active” is vital to their workflow, and to insist that they are mature enough to use sharp edged tools without cutting off their fingers. So, Trimble has left this as a matter of guidance and advice rather than hard-coded rules. There are excellent extensions and plugins, such as TIG’s layer watcher to enforce the advice for those who are less self-confident.
As a software developer, I am always amused when a non-developer asserts that something must be simple to change. I don’t have access to the source code, so I won’t pretend to know, but from experience I can think of lots of possible ways such a change might be far more difficult than you imagine. And even if it is simple inside SketchUp, it would likely break a large number of extensions and plugins.