Ovinomancer
No flips for you!
You're not totally wrong: wielding a sword once you and the monster contact, he has to take an OA to get to the squishy (S). Which is why using a reach weapon without feat support is generally not a great defensive tactic (Reach is great for hit and run battlemasters or rogues though).
However, with polearm mastery the trade off here is before he gets to the Squishy he's got to go within 10 feet of you, triggering the OA. So in this instance, you're "Fending him off" with the threat of an OA (As soon as he steps on any 2, you attack), rather than "Locking him down" with the OA threat. It's essentially where the monster pays the Pain toll.
Now where the Polearmer really shines is multiple enemies: no one can run around him because he's got such a huge hit box....again, they all have to pay the Pain toll to get by (Unless they all charge at once). Again in your example you are right that the monster only pays the pain toll once. However, if your squishy takes a 5 foot step back, now they are paying twice if they don't move fast. Take the Sentinel feat instead of GWM and your reaction is pretty much getting used every turn because, with good positioning, everything need to run past you and you stop them dead. 5 feet away. Then you attack with reach and move backwards. It can be pretty devastating.
Against a single enemy, yes, it works well. Numbers make it much less useful, though, as you're still limited to 1 reaction a round.