There needs to be significant changes before Sea of Thieves will ever have the playerbase it used to have.
Elite Dangerous has nothing stopping anyone from opening fire on other players. Anywhere. At any time.
But there are port authorities. They are decent pilots with decent ships and they come in teams. In that game I run a freighter. It isn't meant for dogfighting and certainly can't outrun the typical port authority ships. But even then, one on one, I can usually shoot my way out of a scenario. In a freighter. With no shields. Few weapons. No missiles, ECM, cloaking or anything aside from raw nerve, tenacity and stubbornness. Oh, and a 25mm chaingun.
With Sea of Thieves the law seems to be "pirates do whatever they want whenever they want".
Even from a historical standpoint this makes little sense. Pirates had a code of honor and were usually former soldiers themselves. They often resorted to piracy when contracts with their respective governments ran out and suddenly they were starving.
From a narrative perspective this actually can potentially add a lot of content to the game.
First, have port authorities. Even if they're a small team of sloops it'll force players to do more than "hur hur hur, I waited in port and pew pew with my big epeen". It forces strategy, team cohesion and critical thinking.
Secondly, add something other than pirates for port authorities to protect. A civilization runs on shipments and supplies that aren't universal. NPC merchant ships would provide not only a new target for gains but also encourages teamwork if the fleets are protected well enough.
For that point, assume a merchant vessel is carrying 20,000 gold in goods but is escorted by three galleons, two sloops and a brigantine. Any singular player vessel has nearly nothing to gain from attacking. But a skilled FLEET of players? Even just two or three sloops can easily get the drop on these guys, sail hard and snatch the treasure before the authorities can react. Suddenly there's a reason to get cooperating at least for a little while.
Third, implement something that makes gold worth it. Right now the only thing gold is doing is "unlock these skins!!!!" And "go on more voyages!!!". This gets boring quickly.
Add ship maintenance. Add ship upgrades. Maybe better sails, better cannons, reinforced hulls, etc. Add the need to repair more often due to time. Boards rot. Cannons rust. Sails tear and weather.
Fourthly, add factions and the ability to have multiple ships of players in the same game. I have a team of five, including Siglei, who routinely play. But we always have to pick one person to exclude because the biggest ship is a four man ship.
If we could do something like "make a faction by doing x, y, z and you guys are going to have a high chance of spawning into the game together" then suddenly you have even more reason to encourage cooperation between players.
Suddenly, attacking that team that's turning in their loot isn't such a good idea when they might have six galleons in their fleet total. Or when Port Authorities can easily scuff up an unwary crew. Even if port authorities turns into the vendors not accepting "hot items" that were recently stolen it adds a huge chance for retaliation by the people that did all the work.
I get the idea of thieves. But a world full of nothing but thieves becomes a chore. Many games die due to their player base rather than other causes.