Since I already have this convenient spreadsheet, it was pretty simple to add an "Assassinate" column. You can follow the link if you want to see the details, but what it comes out to is that you are correct: Assassinate is massively better than Action Surge for levels 1-4. Starting at level 5, Action Surge takes the lead, and it stays slightly-to-moderately better for the rest of the game. This is true for both archers and melee.
The moral of the story is that extra attacks are huge. It is very, very, very hard to make a single attack better than multiple attacks. (The dual wielding rogue is technically making two attacks, but you only get to add Sneak Attack damage once.)
Of course, we are getting way into the technical weeds here with an outdated playtest, which is a little silly. I'm willing to bet there have been any number of changes that completely invalidate whatever conclusions we might draw. But for those of us not named Cybit, these are the best numbers available to us.