Instead of focusing on consumer welfare, antitrust law should focus on 'anticompetitive behavior'. In this argument, amazon is the platform through which all other online sellers go to market, like railroads in the late 19th century. Behaving as both a platform and participant is anticompetitive, regardless of whether programs like 'amazon basics' make consumers happy.
NYT article about the author of the original law paper https://www.nytimes.com/2018/09/07/technology/monopoly-antit...
I don't have strong feelings on the issue, but this may help explain warren's stance, which isn't directly related to consumer choice.