This addresses one part of concerns with online commerce, and might well be relegated to a separate shopping application (total segregation of normal Web + commerce activity and resulting data).

It's still only a part of what a major ecommerce vendor offers, other elements being, potentially, supplier relations, purchaser guarantees, ratings and feedback systems, fraud detection (both buy and sell sides), fulfillment, physical delivery, advertising and brand awareness, lobbying, and more. Competition in these areas would remain.

Any browser-inherent features would also all but certainly move more slowly than independently developed capabilities.

Given the present browser market, it would also have to specifically appeal to Google, even Safari and Firefox are extremely distant rivals in that space. Though as noted at the top: a free-standing open shopping/commerce dedicated app might appear.

It's still quite possible an ecommerce monopolist could capture or repel such an initiative, whether in existing or new applications.

All that said, I'd encourage exploration of the idea, it's certainly occurred to me previously.