it's more akin to a sales tactic
compare the shift in preeminence from "free software" to "open source." the pioneers make a moral/ideological argument why doing such-and-such thing is an imperative, get visibility and carve out a niche, but don't achieve mass adoption. bit later, the strivers show up and argue such-and-such actually just makes good business sense, and this proves interesting and palatable enough to get fully institutionalized
or, say, the shift from "gay liberation" to "gay rights." the argument in the 60s and 70s was that we're inherently subversive entities whose lifestyles by their very nature go against the status quo. by the 00s the dominant line is if only we had these and those legal rights, the final obstacles to our complete willing assimilation into the status quo!
as "diversity" continues its transition from ideal championed by believers to administrative function served by a professional class, it will continue to mutate in a way that benefits those who implement it
No, it's not. It's about social justice and corporate citizenship. Viewpoint heterogenity and other arguments of private benefit have always been secondary arguments.
> Notably, institutions like academia and media increasingly consider viewpoint homogeneity something akin to moral purity
AFAICT this isn't increasingly true of either, though it's perhaps slightly increasingly true (with different favored viewpoints) within individual media firms rather than the media as a whole. The argument that it is increasingly true tends to come from supporters of what was, until recently, the dominant ideology in those institutions and all of society, and is more about that viewpoint no longer being as strongly favored than increasing homogeneity.