You clearly haven't seen the swashbuckler. The 5e rogue is a flexible class, there are a number of of different playstyles it can use.
I know about the swashbuckler.
I already explained this. Subclasses lack the power to truly open up new styles of play nor enhance multiple newly introduce elements..
The Swashbuckler can only duel. Rakish Audacity stops working in 2 orcs come at you.
Really, no. That is not what the word "assassin" says to me, and it sounds like you are describing a swashbuckler.
"Assassin" says to me a character who attacks first, kills it's target in one blow, then slips away without any fighting at all. And the 5e assassin subclass is quite good at that.
That's the old concept of the Assassin. The NPC Assassin.
The Modern Media Assassin slips in, target out the target, and kills/KOs everyone on the way out Black Widow, Agent 47, or James Bond style. Assassins have supplanted Pirates as the "Deadly Dex Guy with good Defence" for a few decades now. Just how Pirates took it from Musketeers before them. It's a generational thing as that Swashbuckler role shifted from genre to genre. Even ninjas are tough and brawl more now.
This is especially true in media when Assassins are on teams. That's why people are not getting the concept of the Assassin. The Ranger is the ultimate solo class as well as lorewise
rangers are sneaking around slittting the throats of their favorite enemies on downtime since OD&D. 0e/1e Rangers ambushed and OHKO orcs and gobliniods like it was a joke.
Almost every Rogue sublclass is a loner as well: Thief, Trickster, Scout, Phantom, Soul Knife. They just bring those skills to the party.
The issue is there was never a Pirate or Musketeer class for D&D to pull legacy from. Also it's a tougher sell. And you can't stick Sneak attack on a Fighter without breaking the class.
Again if Barbarian and Bard can be expanded into full classes...