The developers are still free to package it themselves and distribute installers as .deb, .rpm ETC. If they have a dependency not available in your package manager (or to old a version), they can either also send you that, or have you add their PPA. The only significant problem I see is that their is a fragmented market of packaging systems, which makes it difficult for individual developers to target everyone. Of course *.tar.gz is a good universal installer for when someone doesn't use a major package manager.