That bit of dialog, including Tarkin's response:
"Evacuate? In our moment of triumph? I think you overestimate their chances!"
This exchange, coupled with the briefing scene where the Rebels explain to the pilots (ie. the audience) how slim their chances of success are, is proof positive that it was the arrogance of the Empire that led to their downfall. That shot into the exhaust port was one in a million and the entire film hinges on Luke making it, and even then he wouldn't have been able to without Han intervening. They may have won, but it was by the skin of their teeth.
In the years before Rogue One came out there's been this idea that the thermal exhaust port was a "plot hole" when in reality there's well placed exposition that validates it's existence. Thermal management is key to the survival of electronic equipment and so, expelling that heat is necessary. Filling up that port wasn't an option. Even cars need an exhaust to function. Why wouldn't a space station need one? Especially from a source that is generating power/heat for something the size of a small moon? The port itself was tiny (in comparison to the size of the station), it was placed inside a trench (making it far more difficult to access), it was protected by surface mounted gun towers, and it was ray shielded. Four separate layers of protection. The Empire's arrogance made them believe they were invulnerable, even if they knew the Rebels were planning an attack with a tiny fleet of "small, one man fighters." What difference would it make to the Empire? For all they were concerned about, they had the upper hand.
The original Star Wars may be simple in a lot of ways, but if you really look at it, there's a brilliance in that. It sets things up very clearly, without talking down to the audience.
Hopefully those points might help answer your question
Halliwax
Yes indeed. He's so talented and so funny. His Morgan Freeman impression is spot on.