I fully agree that it does not make sense for non-VTK-based software to implement reader/writer for VTK’s XML-based file formats. It’s not just us: I know many software that supports .vtk files but I don’t know any that supports the XML-based formats (unless the application uses VTK).
It would be probably better to just let these XML-based file formats slowly die and move on to a capable standard file format, such as glTF. VTK already has nice glTF importer/export classes, but I haven’t checked recently if they can already save/load all data arrays, support multiblock, etc. glTF is a very complicated file format (much more complicated than VTK’s XML-based formats), but at least it is a widely supported standard, so the time investment can be better justified.
Simple text-based .vtk format would be still useful for exchanging data with small and simple software tools.