The amount of times I've had this argument in the office is untrue. I think the default values aren't stored in the docx file or something like that, but when you manually set a value it does store it in the docx.

Then you have the whole proprietary blobs in a "open" standard to deal with.

The worst offenders are people who format with tabs and spaces and wonder why it's all messed up.

It’s not messed up, though. It’s just set to a different value. If the exact amount of paragraph spacing is important to you, you can either set it before you print, share the file as PDF or use a proper layouting software. This isn’t a Linux issue, you should do the same when sharing a file with someone using MS Office.

You're missing the point, if you get a document from a MS Office user you can't simply view it or print it and assume the result will be what the user intended it to be. Same applies in reverse if you make changes to the document. This makes LibreOffice unsuitable and not a real alternative.

Your yardstick for a usable desktop system is “every detail and default setting in all software needs to be exactly the same as on the Windows equivalent”.

No, the problem is that most people on this post want it both ways, want to say that LibreOffice is 100% perfect and can fit 100% of uses cases and be used for collaboration and at the same time say stuff like you said "It’s not messed up, though. It’s just set to a different value.". Its one thing or the other, not both.

And for what's worth is shouldn't be "set to a different value" because it breaks compatibility and LibreOffice say it does the best they can to ensure compatibility with MS Office formats.