>> The only way for one person to even attempt cross-platform app is to use a UI abstraction layer like Qt, WxWidgets or Gtk. The problem is that Gtk is ugly, Qt is extremely bloated and WxWidgets barely works. So, how you are going to write cross platform apps, then? :D
This is very hard to do in practice for any big enough software, but then using Qt, Java or C# frameworks is probably worth it since their impact will start to be small compared to your application footprint.