Tool For All??
There can be no tool that works for all computing applications. Flutter and other tool kits may make working in a largish area easier as that is what they cover well. But there will always be areas they will not work very well due specific needs in those areas. What Flutter might do is make develop web and generic desktop applications easier, quicker, etc. (no direct experience with Flutter). And this is a large area but it is not all areas. Also, within this area there will be situations were old school methods are still the best.