Seriously WTF. Why wouldn't Emmett Brown say to Marty 30 years later: "Sup man, you were in the past, you actually gave me the idea for the time machine and saved my life with that letter"
And why wouldn't Lorraine think when her child had grown up like: "WTF this guy looks exactly like Marty, the guy I liked 30 years ago and suddenly disappeared."
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30 years is a long time to remember a face. If my memory serves me, Marty never had any photos taken of him in the past, so his parents had to remember his face. If they ever noticed any similarities, they would probably put it down to coincidence because there is no way that their son could have been alive back then (to their knowledge).