Where is it written what "modes are supposed to" be? At the simplistic level they are simply settings on the matchmaking system regarding which players may be instanced with.
The answer to your question is implicit in the statement you've followed it with.
The challenge posed by game, driven by PvE actions, is the same in all three game modes. That other players may pose a greater challenge than the game in Open is obvious, understood, and irrelevant to the game of those playing the PvE game.
The presence of allied players typically makes PvE interactions considerably easier (deliveries go much faster with wing beacons that can significantly cut SC travel times, and NPC ships fall at higher rates, with even lower risks, when multiple CMDRs coordinate), while the presence of hostile CMDRs can make those same interactions much harder.
Your attempt to separate the PvE game, or other players, from the game is nonsensical. This is a multiplayer game and other player characters being present are an explicit feature of two of the three modes. No matter the mode we're all contributing to the same setting and all the modes skew the frequency and magnitude of those contributions by allowing other CMDRs to function as assets and obstacles.
If the goal is for the modes to be fair, and I think it should be to
avoid perverse incentives that detract from player enjoyment, ignoring the different circumstances implicit to each mode is
not the way to go about this.
If one is intolerant of direct player interactions in general, they should be able to select Solo and know they are not being disadvantaged in their ability to contribute influence or merits to the setting, by overly large incentives for PvP in Open, nor benefits from direct cooperation from wings that are possible in both PG and Open.
If one wants to play with their friends without having to worry about PvP, the network issues they will eventually encounter should not penalize their potential contributions vs. Solo players.
If one is tolerant of PvP, or even seeks it out, they should be able to choose Open and do so without sacrificing any ability to contribute to the underlying setting.
Allowing enemy CMDRs to be vaild targets is a step in the right direction, but to truly balance the modes with respect to even the 'PvE game' requires more specific tailoring to each modes advantages and disadvantages.
It's like that semi-popular 'equality vs. equity vs. justice' metaphor...
What we have is 'equality' in that image. All contributions from all modes treated the same, despite the circumstances implict to each mode that all but mandate different outcomes.
What I'd like would be 'justice', where every mode offers fundamentally the same experience and difficulty. Where NPCs are no more or less of a threat, and no less useful as allies, than CMDRs can be.
What I think would be pretty easy to achieve would be 'equity', where the differences between modes are acknowledged and accounted for by dynamically adjusting contribution based on circumstance.
