I'm going to have a bit of a hot take here, but I don't think the Sovereigns are going to be a super big deal, honestly. In the grand scheme of things, it's really not going to save that much time, compared to selling to individual trading companies. Will it be an ease of access thing? Sure, I'd agree with that, but again, I don't think it's going to be this game-breaking thing like people think.
As it is right now, the only outposts (minus GS for the time being) that take a bit more time to sell on, are Daggertooth, and Ancient Spire, respectively. Daggertooth, the trading companies are very spread out, and with big hauls takes forever to sell on, so most people avoid selling there, unless it's a dead server (or at least my crew does). Pretty much everything there is a decent bit of walking. Ancient Spire isn't all that bad, but it made the list because of having to climb the ladders, or take the long away around to get to the Athena's Fortune turn-in.
Let's look at it this way; currently players will often do treasure drop-offs, when being chased. As it is, if a ship is chasing another ship with treasure, and the fleeing ship goes to drop off a high-value treasure, the chasers have to cannon over to the outpost, guess what type of treasure is being sold and where, and actually make it in time to whichever trading company turn-in spot, before the sellers get there.
With the Sovereign building, it's one location on each outpost, and the locale itself is a tent-like structure, with limited-to no visibility of the inside, from the outside. This means that tuck plays will 1000% be made on these, and I would actually argue that turning in here would be a bit more difficult in many scenarios.
Turning in with the Sovereigns not only eliminates the need for the chasers to deduce where the fleeing crew is selling, but, also provides better potential opportunities for yoinks. I can see people tucking in the tent itself, or up in the crows nest-like structure at the top. This is all not even mentioning keg plays, either. The tent itself looks quite able to hide numerous gunpowder kegs with ease, and an unsuspecting crew would be none the wiser.
Yes, it's basically a reaper turn-in on all outposts, but you are trading an ease of selling for an ease of counterplay.
TLDR; it's harder to make outpost steal plays now, because people have to deduce where you're selling. With the Sovereigns, it's easy to figure out where someone is selling, and you can coordinate an attack better.