Many ramps are built with federal money. If they close them down, they have to reimberse the feds. That happened when blago closed down Illini State Park, but that was access to the Illinois River. They had to leave the ramps open, along with an access road. However, they weren't required to maintain the road or the ramp, so you are traveling the road and launching at your own risk.
Sangchris is a privately owned lake, built by a utility company, and is operated under a reciprocal agreement between the Utility and DNR. If that agreement is not renewed or cancelled, I guess the Utility could close the lake if they wanted. With liability concerns, I would expect that closing the lake would be a pretty good possibility. If the ramps were still open, I guess you could launch, but you would be tresspassing, so you could be arrested and your vehicles impounded.
With that being said, when Comm. Ed. owned the lake, even before Conservation had an agreement, they didn't care. If you could get a boat in the water, you were good to go. Of course, liability lawsuits weren't as prevalent in those days as they are today. Who owns the powerhouse now? It would be up to them.