[michaelmrose]

Neither gtk3 nor qt5 are particularly heavy as far as resources usage. Keeping older versions of libraries like gtk is the cost of keeping a rapidly shrinking cast of existing apps working. Note that qt4 and gtk2 are 7 and 8 years old respectively and can be used with current OS if needed by legacy apps. If your distro opts not to support it the dev or packager can ship it with the application. See flatpak or better nix.

The storage consumed is reasonable in terms of modern sizes storage.

Purely from the perspective of the end user I'm not sure I understand what the difficulty is.

The user doesn't care if the app is gtk 2 or 17. From the user's perspective they click install and an icon appears and their free hard drive space goes down a tiny amount.