The only differences between a main menu Open PvE option and Mobius is that Mobius isn't advertised in game, it's more tedious to use and it has arbitrary limitations (like the 20,000 player limit).
No. One other very key difference is that Mobius is moderated as to what counts as PvE and what counts as PvP by a real person.
That's hugely important because the line between PvP and PvE in Elite Dangerous is not at all clear-cut, and indeed the same physical actions can be PvE or PvP depending on context.
For an example, if I fly up to a random miner in an Anaconda and launch a swarm of collector limpets to steal their fragments, that's PvP. If I do the exact same keypresses to someone I've agreed out-of-game to help refine their ore faster to help them get started on their mining career, that's cooperative PvE. The game can't tell the difference.
A more likely and trickier example: try to find a set of collision rules that doesn't
- allow players to be rammed to death by other players
- allow players to be blocked inside a station by other players
- allow players to trick the station into firing on another player
In Mobius it's easy - the rule is "no exploiting the collision rules to harm other players" ... but you can't tell a computer to enforce that as it's way too imprecise.
Frontier should absolutely:
- increase the maximum PG size
- make the PG management interface easy to use with 1000+ player groups
- make the PG management interface delegatable so that it's not all on a single group owner's head
- allow PGs to be advertisable and searchable in-game
- allow a single account to manage multiple PGs
- allow PGs to be named something other than "account owner's name"
- probably way more stuff I haven't thought of but which they could ask big PG owners for ideas on
Trying to design an PvE computer-enforced ruleset would be a complete waste of time that would just have certain people finding more creative ways to PvP and give Frontier an endless series of bug reports, support calls, and forum threads to deal with or ignore.