You aren't required to, you can use system chat or the squadron tools. Many things in Elite are easier for every player with third party tools. I wholeheartedly agree it would be nicer to have more in-game solutions available, absolutely.
I agree, and yet the game rarely serves up 'organic moments' even in Open. The number of interactions available between players is fairly limited and depends at all times on the goodwill of all participants. Out of interest what kind of organic moments in particular do you envisage when you say this? I'm not being snarky or hiding an agenda with this question, I'm genuinely curious about what you want from it.
I'm honestly not trying to do that, quite the opposite in fact. I started by trying to show that a PvP flagging system in Open would have downsides, important ones, including the suspension of disbelief angle that you rather flippantly dismissed. We've drifted from the precise suggestion in the OP but it is still colouring the conversation. FWIW I realise that you in particular aren't suggesting a flagging system - but it's important to me that people understand the stakes instead of replacing one perceived bias with another.
Beyond that I am just stating that Elite's multiplayer doesn't flow particularly smoothly anywhere, Open or otherwise. This isn't to tell you to shut up and stop asking for an uncapped mobius option - which as I've said is a reasonable request. There are however broad misconceptions regarding how this game works that I genuinely think it's helpful to dispel. These misconceptions can lead people to be exaggeratedly frustrated by the status quo, and by players whose preferences do not match their own, when in fact we're pretty much all labouring under the same basic limitations regardless of mode.
System chat and squadron tools do not permit you to enter into a shared PvE experience in Elite Dangerous, currently.
You first have to join a private group, which in Mobius' case involves their third party website. Many other private groups require you to join their discord, instead.
Many players don't even know Mobius exists. Newcomers to the game want to get in and experience the game, not spend hours doing things extraneous to being inside the game.
Even with the constraints of being in Mobius, I've had memorable 'organic moments'. The simple impact of seeing other players existing in the game cannot be understated (though I think typical MMORPG games take this too far, with hordes of players running around towns jumping and waving their arms...but I digress.) Not having to worry about goodwill of participants is one reason I much prefer PvE experiences to PvP - generally speaking, players that seek out PvE intentionally have goodwill to begin with anyway.
Regarding what 'organic moments' are, I would point to Community Goals as a great example. Being able to greet other CMDRs, join up together if we feel like it, forming groups and cooperation to achieve goals, is a lot of fun. Some of my best memories on this game are of meeting other pilots in Mobius during Community Goals.
Just the other day, I met a couple pilots who happened to be in HIP 36601.
I was surprised and pleased to see that there were still people in my Mobius private group that are active and playing after all these years. (EDIT: I've
just figured out that "System" wide chat applies across all game modes (when did
that get put in?), not just to the private group you are in. Oh well, it was still a moment of meeting somebody.)
One offered me a ride on his Fleet Carrier, but I happened to be going the other way and declined.
It's worth pointing out Fleet Carriers have opened up a whole series of avenues for player interaction (I myself have been greatly enjoying meeting the P.T.N.), and there's plenty potential (as with everything in this game) for Fdev to add onto that. I can't stress enough how much I'd love to be able to see other P.T.N. pilots ingame without having to be in the same private group.
I spent many years, once upon a time, playing a silly game called Puzzle Pirates. The strength of the game was not in its art (quite dated) or mechanics (monotonous and simple, being puzzles) or even setting, really - it was the ability to commune with other players, voyage together, form crews, build communities, and create lasting friendships. The game sailed on
much longer than it ever really deserved (due to several highly questionable developer decisions and then multiple handovers and other failures), purely on the strength of that aspect.
That kind of magic can't happen when you can't even encounter the other players safely or without lots of pre-arrangement.
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For the record, I wouldn't approve of a 'flagging system', that brings too many flaws and complications. All I'm for here is an official PvE mode that appears right on the first splash screen you see as you open the game, right alongside Solo and Open.
I can agree that the multiplayer experience in general could use further development and smoothening-out.
The thing is, many people like myself have been frustrated about the neglect of an official PvE stance because it's been an 'obvious step' that we've wanted to be taken for at least the last 4 years, and probably ever since the Kickstarter days (though that was before my time). So I myself don't particularly believe that the frustrations have much to do with misconceptions of what's possible with P2P instance infrastructure and so on.