Although Smallville's inability to use Batman as a character on the show was disappointing, it also served as proof that the iconic vigilante is one of DC's most replaceable heroes. Smallville's 10-season run saw the Superman prequel series become one of the Man of Tomorrow's most comprehensive iterations despite never showing Clark Kent don the iconic cape. Smallville's status as a Superman origin story allowed for the use of some of DC's best characters, including several future Justice League members.
While Smallville saw Clark Kent team up with Aquaman, the Flash, Green Arrow, Martian Manhunter, Cyborg, Black Canary, and more, Batman was never among them. The reason for this is believed to be connected to the production of the Dark Knight trilogy, as the three Batman films were all made during Smallville's run. Yet while it is certainly lamentable that Batman was never able to appear on Smallville, his exclusion from the Superman-centric series was ultimately for the best.
In fact, not only did Batman's absence help Smallville to become a more cohesive Superman origin story, but it also highlighted a key issue with the character: he's replaceable. After requests to use Batman on the show were repeatedly denied, Green Arrow was brought in instead to fill the gap of a non-superpowered hero to work alongside Clark. By introducing another billionaire vigilante who lacked superhuman powers, Smallville inadvertently pointed out how easy it is to fill Batman's place in the wider franchise.
Smallville's Oliver Queen/Green Arrow filled the role of Clark's friend and rival perfectly, capturing the tense and occasionally volatile relationship that Superman traditionally shares with Batman. Although Smallville's Clark never met Batman during the show's ten-season run, practically every story that their partnership could have explored was written with Green Arrow in the Dark Knight's place. Not only did Green Arrow fit the bill perfectly, but in some ways, his presence surpassed Batman's theoretical role within Smallville's universe.
Batman being such an iconic character also comes with a downside - in as much as it's difficult to satisfy every idea regarding how he should be portrayed. Green Arrow following such a similar narrative arc (orphaned billionaire CEO who takes up the mantle of a masked vigilante) allows for an exploration of the same themes without the necessary pageantry involved with introducing a character as iconic as Batman. Not only could Smallville address the stories that arose as a result of Clark's work with more human heroes, but he was never forced to compete for the limelight in his own show.
Interestingly, Tom Welling's Superman eventually met Batman in Smallville's season 11 comic book arc, but he never appeared on the show. This allowed Green Arrow to step in and fill the Justice League's non-superpowered vigilante role, making Batman's absence barely noticeable. Thanks to the similarities between Batman and Green Arrow, Smallville functioned just fine without using DC's other most iconic hero, proving that Batman is far more replaceable than his reputation implies.