But there are players that can't have high-quality connection to game servers whenever they want (like players physically far from game servers and/or have poor Internet connection), and the stable online time they have may not be enough to carefully design and test their creations. I don't think we should just tell them to buy some VPN stuff or to play another game.
This is, however, what every single successful MMO tends to tell players. "If you don't like certain features of our MMO, we can't force you to play. If you don't have a good internet connection; nothing we can do to help you." As such, if the desire is to have an offline mode for those players, the argument is that such a mode devalues the online gameplay component of the MMO. They cannot please everyone. In addition, those players who would stay offline, and those who would want to test efficiently, would all seek this offline mode, and the MMO component would not have the best chance for success. There is value in forcing your players to connect and play the game through as you the developer see is necessary to maintain the MMO component. If everyone could be satisfied, there would be value in satisfying anyone.
"Not everyone can form a 40-man raid for WoW raids," does not seem like a valid argument for making WoW have an offline component. The whole feature of the MMO is that you're playing with a lot of other people. That is the main value of the game - not any individual feature otherwise. "Sandbox" by itself does have value, don't get me wrong - building and creating in your own offline space is a tried and true successful model that many games have already done.
The reason I'm posting a lot about this is because all evidence shown to us so far from the devs points to their desire to make this game an MMO. If this looked to be another Space Engineers just better... I wouldn't see the value in trying to expand on the reasons other successful MMOs succeed; it wouldn't be one. Starbase is at a point where it does have to choose what it wants to be. The Steam page shows Single-player and people are expecting things. Mods on Discord are saying there will be single player creative mode still. The game needs to define itself going forward. And it could indeed just have an offline creative mode and individual servers - this is totally a possibility and I don't think any of us who are already emotionally invested in Starbase would not buy the game from it. It just wouldn't reach that full potential of an MMO. It wouldn't have the chance become equal to EVE. It wouldn't have the chance to surpass StarCitizen in gameplay and delivery. It would be a pretty good game. That could be good enough; to make a game that tries to satisfy everyone. For an MMO, there's lots of evidence showing trying to satisfy everyone is just not a very successful option. I see a lot of lost potential if the average path is taken, when they've done so much groundbreaking technical work to get to this point. They need to see what could be, and that it is in reach - in fact from what I see, it already exists - it just needs to be committed to in their minds, one way or the other.
There was a ~40 minute video from Anna from Frozenbyte showing some of Starbase development history. It was really good information. The beginning of the game development was nothing like what exists today. They didn't have this specific end-goal in mind at the beginning of development and I think they're still shaping what the game is supposed to be. The initial idea was far less ambitious, and it was through taking calculated risks - and devs delivering on those risks - over the development time that led them to this large space-MMO position. I feel they are poised to achieve more success being "The truly great space MMO" than "Another spaceship builder."
No Man's Sky was sold as being a multiplayer single infinite universe and delivered just a bunch of people working in offline servers but connected to a discovery server, with nothing you did in your game actually affecting anyone else, and even then, there were billions of planets so it didn't even matter. With all that, they sold over a million copies with some deception and a lie. There was no 'one universe' in NMS, so nothing players did impacted another player. Although NMS has made some amends, and plenty of people still play it, the push that got them all the sales initially was for that shared open universe MMO experience. Compared to initial hype, there's few playing it and few who come back to it for unique gameplay, not because the game isn't good, but because there's no one else there. Because it's empty of the things that matter - people.
I do see what Starbase has in front of itself and I think with the right drive to make a proper MMO, Starbase will capture a huge part of the space-MMO market, which is massive. If they commit to that, there's more value in reaching for that holy gail of MMO development, than offering individual offline servers to test build spaceships. The game is much more than that even in its current state. Many of the arguments here are pointing out how devaluing the online component doesn't do the main game any good.
As far as third party tools and simulators - those are almost always created well after a game's launch. Some basic tools have already been created for Starbase. Those tools will pull players out of the game itself, into a third-party tool, similarly to an offline sandbox. If the argument is that people will develop ship building simulators, then that is work the devs didn't have to do, and the devs maintain control of their online game experience and all the potential that brings. With no other outlet than the online component, the initial social-chain-reaction of people in a brand new MMO working together and interacting together is the critical-mass to success. If most of the players instead dipped away to an offline mode because it was available day 1, the live game world has little chance of being something great... and the devs relinquish the value that comes in scarcity of gameplay where the only place to get Starbase gameplay is inside the one fully connected online MMO universe that actually works.