The problem is that Open mode players don't feel rewarded for the extra risk it takes to play in that mode.
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If I'm not mistaken, the '1st Triple Elite Commander' contest had as a sine-qua-non condition that all Rank points should be earned in Open mode. So even the very own game designers/developers did acknowledge that playing in Open is inherently more difficult/has more risk-merit than playing Solo. At least, enough extra risk to award a 'true' or 'valid' 1st triple elite status.
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However, that extra risk component is currently utterly ignored/dismissed in every aspect of the game. Thus, most open players feel not properly rewarded, and if you ask my opinion, for a valid reason. The same underlaying principle which prevented a solo player from getting '1st Triple Elite' still exists, but FD chose to look the other way instead, not acting coherently.
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FD doesn't want to change Open-Group-Solo switchability to give players freedom to chose? fine, it is a valid design choice and one that will attract/make most players happy. But most of the problem will be adressed just by gaving Open mode players that bit 'extra reward-acknowledgement' for taking the 'extra risk'.
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For example, you may earn Rank and Reputation faster while in open mode. Or your missions/cargo may provide extra reward (rank+income) when played/traded in open. Imagine your cargo or your active missions having an 'open' flag attached to them. Likewise, stellar bodies discovered/explored in open also get the 'open' flag. However, if you switch to solo at any point, any active mission in your transaction tab, any cargo in your hatch and any stored nav data permanently loses the 'open flag' and stops providing the 'extra open bonus', even if you reconnect to Open later. You would still get the full reward/Price for your operations as you currently do, only without the extra bonus.