By "C is so old" I mean it lacks a lot of features modern languages have. Proper linting, code formatting, dependency management, version management, virtual environments, modules. Yes, you can solve some of it with docker but it's terrible compared with Rust for example.
By "it doesn't translate well to Rust" I mean that GObject doesn't translate well to Rust structs so you end up with weird structures split into multiple modules and terrible code overhead. Compared with modern UI frameworks it's just not ergonomic to work with.
Yes, I know GTK supports multiple platforms but if I want to develop for desktop and mobile I had way better experience using Tauri+Leptos. It's not just about having some bindings and some docs for it. It's about how much effort does it take to set it up and figure out how to implement specific functionality. Good docs, good tools good compiler and readable code for the framework help a lot.