Ryan got the first tax reform bill since Reagan through the Congress and did so by dragging Trump along and keeping the reactionary wing of the GOP under check. What more of a mark could he make as Speaker (especially having to work with THIS President) than that? It is not as though Gingrich, Hastert, or Boehner accomplished anything more significant or of greater appeal to GOP voters.
If he bows out now he has options politically and otherwise. If he remains, he is tied to the current administration. If Trump suddenly becomes a President of legend (cannot calculate the odds against that, cannot count that high) then Ryan is done politically. As an outsider, presuming Trump does not become a legendary President, Ryan has options.
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