I don't see why it would be any different on Linux. There will always be legacy applications that will stay on old toolkits and they cannot fully benefit from

I don't see why it would be any different on Linux. There will always be legacy applications that will stay on old toolkits and they cannot fully benefit from sandboxing. However, a lot of widely-used Linux applications that are on older toolkits are currently working on upgrades. E.g. Inkscape and The Gimp will be Gtk+3 applications. There are often other carrots, such as proper support for scaling for HiDPI screens, etc.