One thing I never see discussed is if Bitcoin were to truly become mainstream, it would require "the mainstream" to purchase all of their Bitcoin from existing holders. In essence, all of the people who got in early get to decide who does and does not get to participate in this hypothetical Bitcoin-based future.

Setting aside all the other issues with Bitcoin (scaling, energy use, volatility) it seems like this would be a major issue if Bitcoin was ever on the precipice to being considered "mainstream". People would most likely strongly object to having to go purchase bitcoin from a bunch of people who bought in early in order to continue functioning in their lives. It is one of the big, largely undiscussed, reasons I believe that Bitcoin will never truly catch on.