I haven't done the math, but 2 SAs per round as the default seems like it would vastly favor Rogues as the top-tier DPS machine.
First off, let me reclarify we're talking feat-enabled games here. Good.
Now then. I am of the firm belief Rogues should be DPS machines. Skill use is vastly overrated by a dev team that perpetuates the fantasy that the three pillars are in any way equal. Just look at any published module to instantly see that the game is maybe 80% combat and 15% exploration. The times you make a social check that actually matters can be counted on one hand, for all books together! So putting even 5% on social is generous.
Now, your question:
Why would anyone ever play a fighter?
Because you have vastly better staying power? You don't need to set up special attacks. You have the best weapons, the best armor, and the best combat feats. You also have more hit points.
Also of note: it's a group game. I know there are abilities that rely on Rogues sneaking about, but we've found that in practical play, they're close to worthless.
Nobody wants to sit on their hands while the Rogue solo-scouts the dungeon. (Plus, never split the party)
So the game is about a group of people that make noise and light.
The Rogue should work just like all the other classes: in the context of a group activity.
Giving them what they already have (a second SA) is the best suggestion to answer the OPs query.
It makes them a valuable ally. Much like a Wizard, someone worth protecting.
Without a second SA, the question I'm asking myself is:
Why would anyone ever play a rogue when you can play a fighter?
Thanks to backgrounds and flexible skill acquisition, one of the party fighters can become the swashbuckler. The secret is that skill DCs are very weak in this game. You don't need Expertise to stealth or find traps.
And you're much better at doing the 80% activity: killing monsters.
As for the subclass abilities you get at high level, they might need tweaking. At this stage however my main concern is this:
Rogues need a reason for being in the party. In games with feats, that reason is two SAs already at low level.
That's the message I'm sending out.