Malonic acid and its derivatives have their own demand as esters and neat acid apart from conversion to 1,3 propanediol. It's possible to catalytically oxidize

Malonic acid and its derivatives have their own demand as esters and neat acid apart from conversion to 1,3 propanediol. It's possible to catalytically oxidize 1,3 propanediol to malonic acid, so they are somewhat competing in the same space. The "clean" advantage of these fermentation processes is IMO greater than the "sustainable" part. You could use renewable energy to synthesize all the upstream precursors of traditionally-synthesized malonic acid (e.g. monochloroacetic acid, hydrogen cyanide) from air and seawater and it would be perfectly sustainable. But the intermediate products are hazardous to handle and there's a large waste stream of aqueous salt solutions and organic solvents. Eliminating hazardous intermediates and producing waste streams that can be handled by simple wastewater treatment makes it cheaper to build and operate facilities for these chemicals.