Right, so "write an essay following X structure that's Y paragraphs long as defined by the structure" (eg three arguments for and one countered argument against, plus an intro and conclusion) is fine. "The paper must be at least 6 pages long" is not. I'd much rather read and write "this uses Mouse A rather than B because A was much cheaper for similar functionality" instead of "the reason for which the first discussed mouse, Mouse A, was chosen to be used in this project instead of the alternative, Mouse B, was due to a cost-benefit analysis. The first mouse performed various tasks at near or surpassing ability when compared to the outputs of Mouse B; with this, and the significant price discrepancy heavily in favor of Mouse A, it is clear that the minor detriments in functionality shown by Mouse A are easily outweighed by the the more potent upside of being notably less expensive than Mouse B."
You may notice that the second example is TERRIBLE writing: too many modifiers, uncomfortable sentence structure, repetition of full names instead of pronouns, uses and defines "cost benefit analysis" instead of... not doing that. But it's longer! Easier to fit that length minimum! If you intend to require a depth of argument or a number of pros and cons, say you'll grade on that.
1.) I have never seen essay about mouse selection. While possible, that would be purely exercise in structure. Structure exercises exists, but again rarely about topic like mouse selection. Just a bad match to format.
2.) The style of writing in your second example would not get you good grade in writing course. Simple as that, while long enough, it is not good enough.
Depth of argument is not contradictory to good sentence structure. Or to good overall structure. The essay is not actually graded on smartness of argument, but on whether you present them well.
And yes, writing courses focuse on writing elements of text. Just like programming assignement focuses on coding and less on your work being useful.
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What you did there is trying to hack writing assignment. It typically does not work, teachers are typically not that dumb. They would however simply call that "bad writing" or "did not put in effort" rather then flattering "hacking the text".
You may notice that the second example is TERRIBLE writing: too many modifiers, uncomfortable sentence structure, repetition of full names instead of pronouns, uses and defines "cost benefit analysis" instead of... not doing that. But it's longer! Easier to fit that length minimum! If you intend to require a depth of argument or a number of pros and cons, say you'll grade on that.