An SBOM as part of a contractual requirement when purchasing software seems totally reasonable to me if the receiving organization already has the practice of checking a lot of versions and making sure they're sufficiently up to date. But the hard part there isn't the creation of the SBOM, it's a) actually using the SBOM, b) having enough contract power that if the SBOM turns out to be incomplete, out of date, or a lie, the purchaser can do something about it, and c) the purchaser doing something about it.
Nutritional labels only work in practice because a) plenty of people read and care about them, b) there are regulatory agencies that set standards and enforce compliance, and c) if they are too far off, an expensive class action suit is a real possibility.
My concern with starting with SBOMs is that since they're orders of magnitude harder to read and evaluate, and since many, many companies are already bad at tracking their own software patch status, approximately nobody will actually use them. Again, I look at the Equifax breach: it happened not because they didn't know what a vendor was up to, but because their internal processes weren't sufficient to turn knowledge into results.