The only bad thing about the original continuity is that (1) TOS did not age well at all, and younger audiences have a hard time connecting with it, (2) TNG was really great, but I think they've squeezed everything out of it that they could, (3) DSN and Voyager didn't do very well and are often less remembered, (4) Enterprise was fun, but was kindof gimmicky, and hardcore fans don't accept it into the lore.
All that being said, they didn't have much room to build on anything in the original continuity and make any actual money off of it.