As you have highlighted, 1983 shows that principle quite clearly. Piquet relinquished the lead of the last race to his team-mate once Prost was out and he no longer needed to win the race.
At least 1983 is an easy one to adjust because it is obvious what would have happened. Something like 1977 is somewhat trickier.
I once fell into this trap myself when trying to work out who would have been 1956 champ if car hopping wasn't allowed. I soon gave up. Way too many guesses and assumptions had to be made, it was a total waste of time.