The PJM grid did a study and concluded that taking coal and nuclear off their grid and replacing them with LESS expensive generation from combined cycle gas plants would not impact grid integrity. This is just a guarantee that you will be paying higher electric bills to keep these forms of generation operating.
http://www.pjm.com/~/media/about-pjm/newsroom/2017-releases/20170330-pjms-evolving-resource-mix-and-system-reliability.ashx
(Valley Forge, Pa. March 30, 2017) The PJM Interconnection system can remain reliable with the addition of more natural gas and renewable resources, an analysis released today concludes. However, the report notes that an increased reliance on any one generation type brings resilience risks not accounted for under traditional reliability standards.
The report, PJMs Evolving Resource Mix and System Reliability, responded to questions about the effects of fuel diversity on reliability. PJM stakeholders had questioned whether the system is losing too many traditional resources as coal plants retire and nuclear owners consider their future.
This analysis underscores our responsibility to continue to operate the system reliably, and explore the role of resilience, the ability to tolerate unforeseen shocks and continue to deliver electricity, said PJM CEO Andy Ott. Different resources provide different reliability attributes, though new technology or regulations have the ability to improve those capabilities.
PJM needs to work with stakeholders and the industry to determine whether markets and operation structures need to shift to make sure that necessary levels of generator reliability characteristics are maintained in future resource mixes.