IMO, it's a bit of a stretch, that. Having to invent a particular frame of reference in order to develop and unequal view of the game modes. Taken at face value, the modes are even. Any amount of mental acrobatics will eventually find some manner of "inequality" in anything.
The modes are currently unequal under a population scope of examination.
The modes are currently unequal under a cooperative scope of examination.
If you were to try and break down every aspect of the game under a microscope you are going to find inequality relative to other game modes everywhere. It becomes an exercise in futility attempting to correct or compensate for every minor facet of the game that could be taken as unequal in relation to another.
Far better to simply ensure that each game mode has the same tools, and that all those tools perform equally, and let player agency take its course.
But the scopes you use as examples don't demand any sort of "even ground."
Population inequality implies distributed player preference for mode.
Cooperative inequality excludes solo since it doesn't involve player cooperation. Solo is a mode of player preference of playing alone, and it lacks competitive elements per se. Private has a controlled environment for cooperation, by player choice, without competitive elements per se, and Open has an uncontrolled environment for cooperation, by player choice, without competitive elements per se.
A competitive scope has more of a priority in terms of relative fairness and even ground.
The atmosphere of PP as a competitive mechanic is mutually exclusive to the atmosphere of the game in general, as Sandro implies where he doesn't wish to expand this effect onto anything outside of PP.
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.... only if you are pledged to a different Power. It's down to each player's choice of Power. Similarly, players may choose to engage in PvP in relation to Powerplay is a choice - which generally requires players to play in Open as it is the most likely mode to encounter PvP in. Player opposition is expected. Direct opposition is not required (but is possible).
But then you are merely giving a directionless explanation of player options for PP. Precisely because player opposition isn't required, people have a natural incentive to PP in private and solo, and Open becomes an irrational choice. Those that originally wants to play in Solo and Private have their preferred mode to play in and the competitive edge whereas Open players have to gain a competitive edge by losing their preferred mode of play. That doesn't sound quite proportionately equal.
Direct opposition is not required, no, but it doesn't make it any less of an integral part of the design that doesn't require attention/consideration as any other integral parts of the design.