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.
For multiple lines just duplicate your shapes and group or even join them. For varying thickness you can duplicate your shape and layer them. Rounded corners are supported on every vertice so I'm not sure where are you geting that.
1) multiple outlines: with your strategy, you can no longer edit the shape as a single shape afterwards.
2) varying thickness: it sounds like you're thinking of thickness varying abruptly from one line segment to another, but remaining constant throughout one segment. I would like a feature where the stroke width varies continuously from one vertex to another, that is, get smoothly thicker or thinner.
3) you can only have rounded corners on every vertex or no vertex of a shape. You can't choose individually for each vertex.
I think these are all features that SVG would benefit from adding.