The TIE Defender was an impressive starfighter, but it likely wouldn't have made a difference in the Battle of Endor, which I will get into shortly. There were multiple reasons it didn't enter widespread production with the Empire. It was significantly more expensive than the standard TIE/LN starfighters and TIE/IN interceptors, and there were political factors that held it back for a long time. That said, TIE Defenders remained in service for decades after the death of Emperor Palpatine, which shows just how impressive the design was.

The Battle of Endor, frankly, was a massive stroke of luck by the Rebellion. It serves as a testament to the overconfident nature of Palpatine's Empire. The Second Death Star was nearly complete. The superlaser and reactor were already functional, as were many of the defensive emplacements. All that remained was filling in the main structure. At that point, luring the Rebellion to Endor would have been a master stroke that crushed the core of the group.

However, there were a few oversights, if you can call them that. Endor is home to a fierce warrior species, the Ewoks. They were massively underestimated. If the Empire had deployed a proper garrison of stormtroopers to guard the shield generator instead of the small security force, the shield would never have gone down. If the Empire had deployed ships in front of the Death Star instead of blindly counting on the shield to hold, the Death Star likely would not have been destroyed. If the Emperor had not insisted on sitting in the Death Star to watch the battle, the Imperial forces would not have scattered after the destruction of the Death Star.

The Rebellion forces were vastly outnumbered and outgunned. It was a stroke of luck that the Ewoks provided a distraction while the strike team destroyed the shield bunker. It was a stroke of luck that Arvel Crynyd made it through to the bridge of the Super Star Destroyer Executor, sending out of control into the Death Star. And it was a massive flaw in the Imperial plan to not have a fighter screen defending the Death Star. With a proper fighter screen, the Rebel fighters never get through to the core, the Death Star is never destroyed, the Emperor never falls, and the Imperial fleet lays waste to the comparatively tiny Rebel force.

TIE Defenders would have done nothing to change this outcome, because looking at how horribly planned the trap at Endor was, they likely would have been deployed behind the Imperial fleet and would have seen minimal combat.

Look, the TIE Defender was an excellent platform, between on board deflector shields, hyperdrive, proton torpedoes, and various other functions it was a vastly superior fighter/bomber than anything the Imperial Navy had available during the battle of Endor. The problem with the Defender has been and always will be it's immense cost: in my thesis I estimated that the factory model TIE/D cost 300,000 credits, more than 5 times the cost of a traditional TIE. The truth is, Defenders were too cost prohibitive to deploy in numbers sufficient to turn the tide of the war, especially after the damage caused to one of the Empire's critical TIE production facilities on Lothal by the the local rebel cell.