TVontheRadio nailed it but left out one thing:
Both women ended the Season questioning with actuality thrown in their faces.
Joan sat in the board room, with a new, and might I add unattractive women, working her former position. She has someone taking notes for her nowadays and even though she does her best to take charge of a situation and show a strong stance, Pete Campbell walks out on her like he would never walk out on anyone else on the Board.
As for Peggy, we see her last in her virginia motel about to get the Virginia Slims account. She seems satisfied until she looks out of her window and sees 2 dogs humping. The symbolism here is 2 part:
She's elevated her position and yet she's still sleeping in crappy motels where as Don and Sal were staying in luxury hotels on their getaway trips. Secondly, someone's always gonna get boned, like it or not. With Don, she always knew she would be on the receiving end, but at least it was familiar. At least it was reliable.
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Realo won gold at the Sex Olympics with a BROKEN FRIGGIN NECK.-Voltch