"We can “follow the Bible” back to polygamy. We can “follow the Bible” back to ethnic cleansing. We can “follow the Bible” to a flat earth. We can “follow the Bible” to justify slavery."

All of those are distortions of what the Bible teaches, but they are the stock in trade of the "progressive" way of thinking.  Not once does the Bible endorse polygamy.  It was tolerated and God instituted laws to mitigate the damage.  Ellen White has some things to say about this.  And the Bible does not claim that the earth is flat.  Do you use the term "sunrise" or "sunset?"  Not very scientific.  The Bible speaks in terms we're all familiar with.

Neither does the Bible endorse slavery.  It, like polygamy, was tolerated.  God knew that the principles of the Gospel would eventually undermine it in the church?  Why did He wait so long?  You'll have to ask Him.  But He may say something like what He said to Job in chapt. 38:  "Who is this who darkens counsel by words without knowledge?."