Almost everything you need to create vector art, SVG doesn't support.
Multiple outlines in a single shape? No. Varying thickness in an outline? No. Rounded corners on arbitrary vertices? No. Non-destructive boolean operations? No. I'm not even sure SVG supports paragraphs.
Many of these Inkscape implements as live filters, which are saved as SVG extensions in the XML .svg file that nobody but Inkscape can properly load.
SVG is ridiculously bad as a creation format. It's a good format to export to, but as a backend and it's just insane. It's like using a single PNG file as a backend for your multi-layer 128bpp raster project.
I use Inkscape a lot but I can't help but notice that the best vector art illustration come from Affinity Designer, Corel Draw, and Adobe Illustrator. If you compare the quality of artwork made with proprietary tools to those made with Inkscape, it's very clear that Inkscape severely limits what artists can achieve. You can easily create complex illustrations in other tools that would be a nightmare to manage in Inkscape. Just compare how you clip something in Inkscape to how you do it in Affinity. It's ridiculous how different the two workflows are.
It didn’t need be a proprietary format, it could just as well be a new open format. Inkscape could itself create an open format that they saved to by default and supported many features SVG does not. They they publish the specification for it. Done.
A format doesn’t need to be popular, in widespread use, or proprietary to be useful.
If svgx files or whatever just had some non-svg node in the top level that contained all of the special inkscape-only info, you might even be able to keep it totally compatible with svg itself. (Just obviously, lacking the special inkscape features)
<svg>
<ink:inkscaperoot xmlns:ink="inkscapeschema">
<effect href="#x">
<border strokewidth="1"... />
<border strokewidth="1"... />
<border strokewidth="1"... />
</effect>
</ink:inkscaperoot>
<g><rect id="x" />... regular svg...</g>
</svg>