If you have all 211 cars in free race (as opposed to Enthusia Life), then you've basically given yourself no reason to pursue all S's in DR other than the personal satisfaction in knowing you did it.
If you're talking about EL, and don't have all of the cars in FR, then it is possible without the S's, but would take a long time (randomly encounter car(s) you don't have in FR, beat it/them without getting black flag, encounter more, repeat).