The greatest cross-platform toolkit is Qt that abstracts away all platforms including mobile and embedded systems. Qt also works on top framebuffer on embedded Linux and on other RTOS operating systems. The disadvantage of Qt is that is a C++ library with a lack of C ABI or C api (extern "C") that would allow Qt to be called from other programming languages than C++. That is the reason why Gtk has far more bindings than Qt, although they are unstable due to Gnone breaking changes and lack of backward compatibility.

Most applications just pick Windows win32 API or MacOSX and forgets about everything else and are never released for Linux or other Unices, unless the application uses electron, Qt with C++, Qt with Python or Java SWING like JetBrains IDEA IDE family. The main problem of Linux desktop is the lack of a high level graphics library toolkit with a stable API, ABI and a C interface that does not introduce breaking changes on every release. Unlike Gtk, Qt is more stable, but it lacks a C API and C++ has a fragile ABI not friendly to foreign-function interface or cross-language linking.

Do you see a future where cross-platform could essentially be a set of tools/frameworks to build high performance native experiences targeting the browser as the primary runtime?

Eventually - Google is pushing this with Flutter and Microsoft is pushing this with Blazor.

> There are no great cross-platform toolkits.

True. But there are several entirely adequate cross-platform toolkits. Pick a toolkit, not a platform.

(GTK+ (via gtkmm) user for 23 years, on 3+ platforms)