All of the experts cite the technical, policy and political challenges of a total transformation of the power system.
The technical challenges are looking increasingly solvable, as real world experience shows systems surviving with very high levels of renewables for short periods of time (Germany, Denmark, Colorado). And a steady stream of innovation and progress in PV, wind, batteries, and other technologies suggest it will be easier going forward.
The policy questions are the subject of America's Power Plan, a study of policy barriers to a high-renewable future in the US. The Plan is featured in a regular series of articles on this site, and is available at www.americaspowerplan.org.
The politics issue seems the hardest for now, as incumbent industries are pushing back. But as the market matures, more investors and politicians will see it as an opportunity instead of a threat, and we will hit a tipping point of support.