Assume that the net average return on all companies has stayed about the same. Yet because the wealthy are wealthier than before (economic growth plus rising inequality), they have less need to go public for reasons of liquidity. Thus if they have private information that their private companies will remain profitable for a while longer, they will keep them private and earn those extra-normal returns.
That means on average publicly-held companies will earn lower average returns than before. Which in turn will increase income inequality between the top one percent and the top twenty percent. Which may in turn make this effect even stronger.
So should you buy into the Twitter IPO?