Be Aware of the Makefile Effect

Don't do that! If you're always using the same preamble, you should turn it into a .sty file. Then the preamble of new documents is just

  \usepackage{myessay}

I did that, then I needed to tweak things so I added options, then I needed to use the package somewhere that needed to be self-contained, so I started copy-pasting ;). I've done similar things with makefiles, tox configs, linter settings (all of which started from an initial version I wrote from scratch).

I suspect the real reason this effect exists is because there's copy-pasting is the best way to solve the problem, due to a varying mix of: there being no way of managing the dependencies, needing to avoid (unmanaged) dependencies (i.e. vendoring is the same, only we have a tool managing it), the file (or its contents) needing to exist there specifically (e.g. the various CI locations) and no real agreement on what template/templating tool to use (and a template is just as likely to include useless junk). Copy-pasting is viewed as a one-time cost, and the thing copy-pasted isn't expected to change all that much.

Definitely true for linting. Nothing unlike a linter solves the problems we want the linter to solve.

Then you make a change to myessay.sty and suddenly four of your old files stop compiling.