But that is just the point as long as Gen Meade protected Washington D.C. from Lee attacking, Meade did not have to attack Lee. In fact IF Lee's had assumed a position between Meade at Gettysburg or at Pipe Creek and Washington, Lee would have placed his weaker army between the larger army of Meade and the large garrison guarding Washington itself. With Lee limited to the supplies that he was gathering from the Pennsylvania countryside and the ammunition he had on hand that most likely would have been the total destruction of Lee's Army. It was not a very practical idea. The only way Lee's army could survive was to stay mobile.
The ONLY place that Lee could have been successful in doing what Longstreet purposed was at the Cashtown Gap. If Lee had on July 1st simply stopped the battle there, then burned the town of Gettysburg to the ground and retreated to the Cashtown Gap and set his defences there, the Northern newspapes would have blased the headlines "GETTYSBURG BURNED" and of the complete Confederate victory at Gettysburg and the destruction of the 1st and 11th Union Army corps. Lincoln would have hit the ceiling and demanded that Meade attack Lee's at which time Lee would have had the advantage of the high ground and superior position. While Meade would have had only one avenue of attack. Meade would not have had to attack Lee at Cashtown gap but Lincoln would have demanded it under those the stinging conditions of a Confederate Victory at Gettysburg.
But placing Lee's Army between Gettysburg and Washington as Longstreet purposed? No way. It would have been suicide.
As for the Pickets Charge frontal attack? In our modern day mindset looking upon the actual events it does seem foolish. But in that day and time with the plans of renewed attacks on both Union flanks (with did not happen because of poor coordination of Longstreet and Early) to cooperate with Pickets charge, Lee's plan of attack against the Union center, which had been broken by Wrights Georgia Brigade the day before (July 2nd), but was not supported because of Longstreet failure to instruct all Brigade commands as to the plans of action, was not unreasonable.
Lee's army had destroyed the Union 3rd Corp and parts of the 5th and 6th Corp on the 2nd the day before, and the 1st and 11th Corp on the 1st had Lee broken the center, as planned, the war would have probably ended as the northern people would have been taken by fear of the Confederate army. That is why Lee risk everything he had in Pickets charge. It wasn't as stupid as modern day historians want to make Gettysburg and Pickets Charge out to be.
Just my opinion, but while Longstreet was a good fighter, he was not necessarily a good strategist, nor the panacea as a winner of battles.