If the RSN is a separate fee as it is in pretty much all providers, dropping the RSN only eliminates that fee. That's a fraction of the ~ $35 of the average bill that goes to pay for sports from locals, the ESPN networks, FS1/BTN, and TNT/TBS.
It is also a lot easier to find packages that don't include RSNs than to find packages that don't include the above. You can get packages that don't include the above, but you will also miss out on other non-sports channels like Disney where such packages are offered.
However, Directv could do like some providers do and break out the locals fee separately, and do like Dish does and allow you to get the locals yourself via the LCC and save money. Why they don't do that, and allow many of their subscribers to potentially save $20+ a month, I don't know. Maybe it didn't make sense a decade ago when the locals fee would have been around $2, but it sure does now. They probably would have lost fewer subscribers if people were able to save that much money every month!