No, the Open/Closed nature of the game was not the issue. Forcing players to 'deal' with other PCs would have just driven players away, not significantly booted skill levels. Players who like Solo would not have switched to Open and rushed to interact with PvPers, improving their skills.
The issues are that the AI was too easy, and people got used to it; it's only natural that people would complain when it got harder. This is the main problem and it was compounded by a few factors:
The low difficulty artificially accelerated everyone's Ratings, often outstripping their skill. As the difficulty of our foes being matched to our rating, we are matched against foe which are supposedly better matched to our often artificially inflated skill levels.
The vessels are supposed to be alternatives, rather than a strict progression. This is not a game about the 'next' ship being 'just better', but rather them being generally better. Many players are used to a more linear progression in upgrades where a more progressed piece of kit is 'just better'. Elite doesn't do that, though the poor AI obfuscated the problem (making it appear that bigger was indeed always better) and many players have been very frustrated to discover that that the game's paradigm does not match their own. They have reached the 'end game' without their perceptions being shaken because up until now the more expensive ships have generally been jut flat-out better. Now the AI isn't an idiot, they have had their perceptions shattered.
The big stick approach to the game is expensive. Not only are larger vessels more expensive to buy, they are riskier, due to rebuy cost. Faced with a better AI and more tenable smaller ships, many commanders assuming the 'bigger is better' paradigm was still in force and that any difficulty hike would be matched by their perceived ship superiority, relied on that, and then were upset when the strategy failed... so they tried again... and again, compounding losses and becoming increasingly frustrated. Others recognised the real shift in performance had to come from themselves and switched to smaller, cheaper ships, avoiding higher loss levels.
The thing is that these things will normalise. We will get used to it. New players will enter the game and will increase in combat rank (hence foe difficulty) at a more 'proper' rate which matches their skill increase. They will mature as players in an environment where all size vessels have a place as options and will not be as frustrated when Big Ship gets beaten by Small Ship.
