You've explained the thought process of the "reformers" fairly well, but there is another additional element that's important as well: a sizeable contingent of people believe that advanced math itself is intrinsically bad or negative. Usually this is either because math is fundamentally exclusionary in some core way, or because it's somehow inextricably tied to ways in which equity-seeking populations are oppressed, or used as a tool of oppression.
O'Neill's Weapons of Math Destruction is so popular in academic circles partly because it plays to this mindset.
This viewpoint dovetails with some of the points you've highlighted, for example many believe that everyone is equally capable, but the reason they're not all getting competitive scores with good shots at college is because advanced math (being intrinsically negative/evil) is ruining it. So in their view it is rational to get rid of it, replacing it with something (the data science course) that's not advanced math.
For these people, the goal is not to improve equity by getting everyone up to speed in advanced math. It's to bypass it.