they were killed in combat for the most part. the Germans flew their pilots until they became statistics.
They weren't all killed, many survived.
No doubt many LW veteran pilots were lost over the course of the war but the decline of the LW was not due to the loss of these LW 'super-aces'. As the LW was forced to expand it was unable to train replacement pilots to same level as those early war veterans. The need to get as many pilots / aircraft into the air led to reduced training times, coupled with the lack of fuel and resources for training and the lack of space to safely train replacement pilots, had a greater impact on the fighting ability of the LW. Not only did allied numerical superiority grow as the war progressed, the quality of allied pilots and equipment (especially in the west) increased.
Also, its a myth that LW pilots flew endlessly until the war ended or they were killed. The LW had no rotation system like the Ami's but many a pilot rotated out from front line service, many were offered or ordered to discontinue flying but chose to keep in the fight etc...
When LW pilots, like Hartmann, talk about 'either the white cross, or the Iron Cross' they are referring to the obligation and duty they felt to stay in the fight defending their homeland and families. They were not chained to their Bf 109s and Fw 190s.
Pilots like Wick, Mölders, Gollob, Nowotny, Rudel and even Galland were ordered not to fly combat missions. They did so out of their sense of duty.
Hartmann was sent on furlough in May '43 after it was determined that he was becoming burnt out. Schuck went on furlough four times between April '42 - Feb '45.
Many Allied pilots continued to fly when they didn't need to as well. Yeager, for instance.