The whole usermode side of input handling depends on a project (libinput) that's been developed by a single guy over the last decade. He asked for help numerous times, reminding everyone that libinput has a bus factor of one, and received none. So either go ahead and help, or stop complaining.

https://www.youtube.com/watch?v=HllUoT_WE7Y

I'm pretty sure HiDPI is solved from the kernel's perspective; pixels get to the display, it's just that the (userspace) toolkits have lingering issues deciding what pixels to draw.

Been using both without issue for five years, trackpad for longer.

HiDPI seems to work without issue for me now when using Wayland and Gnome.

Trackpad support on my XPS13 is acceptable now but not macOS standard IMHO. Crucially though I notice it improving over time.

Both of these are userland rather than kernel concerns though aren’t they?

I've never had any issues with trackpads.