A Roadmap for a modern Plasma Login Manager

I would imagine that LIMs becoming more popular could lead to further improvements in that regard, which might actually make more tailored solutions possible at some point. At least KDE already seems to be working on upstreaming this feature in this merge request, so I’m quite excited to see what might be possible in the future :)

because they enable the titlebar to contain useful things instead of being a gigantic waste of pixels

Which can also be achieved with locally integrated menus on SSDs. I use this for example. It may not give you the same flexibility as CSDs, but it does reduce the aspect of wasted space.