I very much work at the coalface here, and "application developers don't need to rely on the distros packaging their app correctly" occasionally happens but is most often about miscommunication. Application developers should talk to the distros if they think there's a packaging problem. (I talk to many upstreams, regularly.) Or, more often, application developers don't understand the constraints that distros have, like we need a build that is reproducible without downloading random crap off the internet at build time, or that places configuration files in a place which is consistent with the rest of the distro even if that differs a bit from what upstream thinks. Or we have high standards for verifying licensing of every file that is used in the build, plus a way to deploy security updates across the whole distro.
And likewise packagers often don't understand that the application has been extensively tested with one set of library versions and that changing them around to fit the distro's tastes will cause headaches for the developers of that application, and that they have a vendored fork of some libraries because the upstream version will cause bugs in the application. It's a source of friction, the goals are different, and users are often caught in the crossfire when it goes poorly (and when each application is packaged N times, there's N opportunity for a distro to screw something up: it's extremely rare that a distro maintainer spends anywhere near the amount of time on testing and support as the upstream developers do, since maintainers are usually packaging many different applications, while upstream is usually multiple developers focused on one project).
Software should be written robustly, and libraries shouldn't keep changing their APIs and ABIs. It's a shame some people who call themselves developers have forgotten that. Also you're assuming that distro packagers don't care, which is certainly not true. We are then ones who get to triage the bugs.
They should, but the world isn't perfect and occasionally you do actually need to apply workarounds (which application developers also dislike having to deal with, but it's better than just leaving bugs in). Distros would run screaming from the bare metal embedded world where it's quite common to take a dependency and mostly rewrite it to suit your own needs.
And I'm not saying distro maintainers don't care, I'm just saying they frequently don't have the resources to package some applications correctly and test them as thoroughly, especially when they're deviating in terms of dependencies from what upstream is working with. And much as the fallout from that should land on the distro maintainer's plate, it a) inevitably affects users when bugs appear in this process, and b) increases workload for upstream because users don't necessarily understand the correct place to report bugs.
