I'm a cartoonist, and I do rather complex vector drawings. It's no unusual for my work to have hundreds of layers, and those layers have dozens of sub-layers. When I opens said files, several layers expand to reveal all the sublayers. I then have to colapse them, one at a time. Is there a quick way to close all sub layers at once?
it has been long discussed in this forum, unfortunately no
the workarounds, either avoid sublayers or move everything under one Mothership Layer then Alt+Click to collapse/expand all sublayers
you're not alone, many of us feel your pain...it does seem like an easy thing to fix, they're either unable to fix it or simply don't want to fix it...or maybe they have other priorities...Adobe works in mysterious ways.
they're either unable to fix it or simply don't want to fix it
Every other Adobe product providing hierarchic foldering for Layers and/or Groups does work as expected by maintaining the last saved layers view upon relaunch. Considering the amount of time (years) this bug has been in place in Illustrator, I have to agree that it's either one or the other. Or worse: they just don't care.