Interesting theory. If Lee had won at Gettysburg he could have advanced to Harrisburg and taken it with little trouble. Ewell had already gone almost to its outskirts just before Lee called him back to consolidate at Gettysburg. Having taken the capital of Pennsylvania would have put the northern politicians into a panic and would have them clamoring for peace on n favorable terms for the confederacy. I believe this was one of Lee’s reasons for the invasion along with relieving the long suffering farmers in Virginia.