The alternatives you suggest aren't GIMP replacements. I am into photography and, for better or worse, only GIMP provides tools for photographers and is an alternative to photoshop. Since Krita and Blender seem to be at a better funding position than GIMP, it only makes sense to donate to it.

Note: I've donated to Krita, GIMP, digiKam and other software, animation projects and libregraphics meetings in the past.