Let me start by saying that the only thing a swashbuckler and a Rogue have in common are light armor. That's literally it. A swashbuckler was a kind of sailor that was a duelist. They could use a parrying dagger, but generally left one hand free to grab onto things on a ship. They did not sneak, back stab, open locks, disable traps, or any of the other Rogue like things.
If you were to represent them as a Rogue, you would have to ignore half of your class features during play.
Not true. I see you are trying to make a worst possible case, but you have far overstated it.
I can think of much worse. This was just an idea off the top of my head.
Not true. Strength is not needed in this build; and Light armour + maxed Dex does keep up.
I guess your image of a swashbuckler isn't fighting rapier-and-knife -- i.e. not two-weapon fighting. So as not to break your concept, you're getting a further AC bonus then from defensive style.
Swashbucklers did not use rapiers. They used short swords or daggers. They also have to be relatively strong in order to do the things that sailors do or they are going to suck at skill checks related to sailing. So we will give them a 16/16 split between Dex and Str:
AC Leather Armor + Dex mod + Defensive Style
AC 11 + 3 + 1 = 15. Not super horrible, but still not anywhere close to a plate mail + shield fighter (20).
No, you have a background that gives you the necessary skills. No extra investment.
A background might give them the skills they need, but a swashbuckler isn't just a sailor. He's a master sailor that spends most of his life on the waters. Looks like they got rid of the feat that gave you mastery over a skill or skills. The closest would we Lucky used solely for skill checks that a sailor might face.
No, you use a finesse weapon -- rapier, which averages one point less/hit.
Sorry no. They used extremely short weapons because they usually fought in cramped spaces and needed to sheath and unsheath their weapon rapidly. They also needed a light weapon in order to swim with it in their teeth, usually a dagger. A short sword or a dagger are more likely.
Short Sword - Attack at 1st level: +4; Damage at 1st level: 1d6 + 3 (average 6.5)
Now you are changing your archetype. Why would a swashbuckler ever tank? I mean, seriously. You've chosen Combat Weapon mastery (or whatever it's called) -- you have all sorts of fun maneuvers to use, all of which are fun swashbuckler-y things.
Go watch some
Errol Flynn movies. He protects his allies all the time, and he's the definition of a swashbuckler.
If you want, you can choose Fencing master as a feat, which gives additional attack/defense builds.
If you do accept the dual wield model, even more exciting things open up -- both with fighting specialty and with the feat. Either way is absolutely viable.
"extremely gimped"? No. "worse at being a tank than a fighter designed as a tank?" obviously, but irrelevant.
This is the kind of stuff that makes me want to play a swashbuckler.
Even accepting the limited version of swashbuckling you seem to suggest, the build is viable and dynamic and potentially fun.
So lets now compare a Swasbuckler to a sword and board fighter:
Swashbuckler (1st level)
HP 12
Str 16 (+3) Dex 16 (+3) Con 14 (+2) Int 10 (+0) Wis 8 (-1) Cha 12 (+1)
AC Leather Armor (11) + Dex mod (3) + Defensive Style (1) = 15
Feat: Lucky
Short Sword - Attack: +4; Damage: 1d6 + 3 (average 6.5)
Dagger - Attack: +4; Damage: 1d4+3 (average 5.5)
Sword and Board Protective Fighter
HP 13
Str 18 (+4) Dex 12 (+1) Con 16 (+3) Int 10 (+0) Wis 8 (-1) Cha 10 (+0)
AC Plate Armor (18) + Shield (2) = 20
Feat: Tactical Warrior (for the ally protective aspect)
Long Sword - Attack: +5; Damage: 1d8 + 4 (average 8.5)
War Hammer - Attack: +5; Damage: 1d8 + 4 (average 8.5)
Now we had to take several sub-optimal choices in order to stick to the concept. It turns out that the Swashbuckler will be less survivable (less hp and less AC), it can't help out its friends by having attacks directed at it, and it deals less damage than the protective fighter.
My question here is why isn't a light armor light weapon fighter supported. Why do they have to give up their concept in order to be able to protect others (like giving up Defensive Style and/or Lucky feat for the tactical feat and the Protector style)?
Now if they put this in the game, it would make it where we could play our light armored and lightly weaponed fighter with no problem and they could still choose to be protective of allies:
Fighting Style
Quick Fighter - Whenever you are wearing light armor you may add your Strength bonus to your AC while you can take actions. While wielding a light weapon in one hand and nothing in the other you gain a +1 bonus to attack rolls.
That would allow them to have a high enough strength where they could forgo the Lucky feat and still be extremely good at sailing skill checks (which many are based on strength). It would also allow them to have a more comparable AC to the sword and board fighter (leather)11 + (dex) 2 + (str) 4 = 17, while still allowing the plate wearing sword and board fighter an advantage in AC. The +1 to attack with light weapons would make up for the lower damage of the weapon. It still wouldn't equal a great weapon fighters damage, but it would come close to the sword and board fighters damage.
There are things they can do to make choices more based on character concept than on mechanical effectiveness. This is just a simple example.