I have come to the conclusion that the only reason why things like flatpak and snap exist is because the ergonomics for creating and maintaining .deb packages is so awful.
Not just that. Application sandboxing is important too. The traditional unix approach of any process run by your user can access all your files is at odds with modern security concerns.
Sandboxing is an issue, though I would argue that sandboxing and packaging are orthogonal concerns, so a new format should not have been needed just for the sandboxing use case. That said, if you are developing sandboxing from scratch pretty much no sane person would choose to use the .deb workflow as their UI for packaging.
I found out when experimenting with it that Snap inherited most of the .deb cruft. As someone less experienced in .deb packaging, I found it very confusing with all the Ubuntu-specific package separations and archaic requirements. If I don’t have a man page, for instance, I shouldn’t have to create one and learn format naggles to meet an aggressive linter’s specs.