The cynic & atheist in me thinks that "Mormon beliefs followed contemporary conservative US attitudes" to be a much more accurate, succinct and possibly honest description of the situation. Racist when it was common to be racist, not racist once that got politically unacceptable.
A similar timeline happened with polygamy/plural marriages. Abandoned when it became politically unacceptable.
Your version isn't actually very accurate. In particular, note that official Mormon doctrine was at times much more racist and at other times less racist than contemporary conservative US attitudes, with significant changes from one "prophet" to the next. I wouldn't characterize it as following others attitudes, but rather as being controlled by the whims of individual men who were all over the spectrum of US attitudes, and who didn't particularly agree with each other.
Your comment on polygamy is also lacking sufficient detail. In particular, note that LDS polygamy was never convenient or accepted by outsiders; whenever it was publicly known, it led to serious community opposition, and probably contributed to their being chased out of Nauvoo [2]. They also officially denied practicing or teaching polygamy, even as Joseph Smith was accumulating a total of 34 wives [0]; he was actually killed as a result of fallout from burning the printing press that was used to expose the practice [1]. Even their "abandoning the practice" was more of a denial that it ever happened, and a weak bit of "advice" to "refrain" from violating local laws [3]. This came shortly after serious attempts by the US government to end polygamy via asset seizures and other forms of pressure; unlike in the 1850s, there were few uncolonized areas they could move to (but some LDS fled to Mexico, including some of Mitt Romney's ancestors [4]). So it would be more accurate to say that the practice was officially abandoned when it became exceedingly difficult to continue. Even so, unofficially within the main LDS church and officially within various offshoots, polygamy continues to this day [5]; official doctrine still teaches polygamy in the afterlife as well [6].
[5] My friend Doris Hansen escaped from fundamentalist Mormon polygamy, and now runs a ministry to rescue other women and children: http://www.shieldandrefuge.org/index.htm . The ministry has its own TV show at http://whatloveisthis.tv/ . The show archives contain lots of interviews with former polygamist men and women.
A similar timeline happened with polygamy/plural marriages. Abandoned when it became politically unacceptable.
I wonder when they'll do same sex marriages.