I don’t think the Sandbox is any more explicitly colonialist or Rightest then a community building game is explicitly Leftist.
You could start a sandbox where the party is a band of refugees trying to survive, or even political revolutionaries on the run. You could play a community building game about building the perfect military ruled exterminationist ethno-state.
The distinction is often not even player goals, and while XP=GP certainly encourages primative accumulation (not capitalism), and may not be ideal for a didactically Leftist game, I don’t think it or the Sandbox are the root of the colonialist D&D narrative. 'Cause I agree that that narrative exists. The issue is likely the perceptions of the designers as largely boomer era American white men raised on John Wayne and Gunsmoke. The difficulty with the Western narrative is not really that it presents a sandbox, but that it treats exploitation as heroic.
Yet the politics of traditional game fantasy has gotten more offputting as it’s moved away from morally suspect Swords and Sorcery heroes to those of high fantasy because it puts even more emphasis on the violent wandering murderer as heroic then B2 did. I think this is the crux of moral play - what the GM and to a lesser extent the mechanics hold up as unexamined good. For me the presentation of the world and providing context for player morality is more important then the techniques.
Yet I also doubt the effectivness of simply changing the direction or target of heroic violence. Players need to be able to act in an evil manner to understand the moral choice involved.
I think what I’m saying is that I don’t think any of this is as simple as replacing the XP system with milestones for good deeds.