I think the custom servers will be a fun venue for streamers. I'm more excited about what new progression options they will add in the future for the actual game.
On the secondary discussion happening in terms of PvEvP, I used to think I wanted PvE servers, but after the past three weeks of what I'd consider very heavy play, I've really changed my mind. The potential danger is a very important aspect of their overall design. I think the group I play with has only gotten into an actual encounter with other player ships two or three times that entire time. Have we seen other ships? Yup. Have we made it a point to avoid them? Yup. Our group doesn't actively engage with other players, as we're mainly there for the co-op experience. We're pretty careful and tend to plan all of our voyages around clusters of islands close to outposts, and STILL make multiple outpost trips. Greed is another major element of their gameplay. If you have more to lose, that's really on you. You have multiple ways to avoid that. Go to a single island then drop off. And while I'll never say anyone can play a game incorrectly, I do think it's fair to say there is a necessary mindset. And while I enjoy the progression, it's actually fairly frivolous. Our group just started Tall Tales a few nights ago, and while doing the first one, we were attacked while we were on the island. We didn't feel like our time was wasted at all. I snuck aboard their ship and caused some chaos, one of my friends lead the two other players on a goose chase, while my other friend quickly solved the puzzle and got what we needed. My friend who led the other two on a goose chase got killed, but then it became a rescue mission for him, as he solo piloted a boat back, while the two of us left at the island hid from the crew. Was it silly? Yup. Did it take extra time? Yup. But, my friend coming up on the opposite side of the island from where the other player ship was, us swimming out to get on it and making a high-speed getaway into a storm to ensure we weren't followed? That's the type of story I wouldn't have, and it made my experience with that Tall Tale something unique, even though the steps are rote.
This isn't even to speak of the other issues that would come from splitting into separate PvP and PvE servers, apart from what has been mentioned. They'd have to start to consider who they cater their monthly updates toward, and then one group would feel left out. They can't just cater to both every month, either, without hiring or updates feeling smaller. It'd have lots of cascading effects, for sure.
Sea of Thieves isn't really a game about progression. The progression is largely frivolous. You don't gain power. It's all cosmetic. I've gotten into the mindset that Sea of Thieves is a story generator. If I lose a cursed skull or treasure chest because I get sunk, oh well. I just don't sit around and let another player ship destroy me anymore. My mind now shifts to, well, if they're going to do this, how can I at least cause some chaos. Maybe it's getting launched at their ship from a cannon. Maybe it's sneaking aboard and blowing up some black powder crates they have in their hull. Maybe it's actually taking a powder crate onto their ship and proceeding to go out in a blaze of glory while throwing firebombs all over the place. But it certainly wouldn't be the same if all I had to worry about were skeleton pirate ships.