DC based and I’m not a commuter but I do vacation down in CHS often enough to have a sense of at least the flight options available. Plus I just flew with a CHS commuter and we discussed the commute. He was using American from DCA to get home.

While CHS-IAD may not have the volume of flights as ATL or CLT, once you factor in flights to DCA/BWI it actually equals or even exceeds those two hubs. Yes you then have to get from one airport to another with BWI being the most restrictive (but it is doable to do it via public transportation the whole way) and yes its relying on other airlines but there are options. And unless one is on the 767/777/787 all three airports are options to begin a trip. So one doesn't always have to commute to/from IAD.

IAD-CHS has also gotten better on aircraft types as well. Used to be nothing but 50 seaters. Now its a mix of mainline (usually Airbus so 2 jumpseats) and 50 or 70 seaters. 3-4 flights a day. Low season sees one airbus and two 50 seaters. Those 50 seaters become 76 seaters during times of greater demand.