… If even one solo player continues in solo without cheating, there is a net win. The more people remain in solo, the more the change would have an effect.
…
You are ignoring the reason for the bonus/compensation. Sandro's suggestion is based on the assumption/observation that players in Open Mode are less effective than players in Solo Mode.
Let's assume player in Open Mode have an effectivity of ½ of the effectivity of solo players (just to make it easier).
Giving open mode players a bonus that results in doubling their effectivity in Open Mode would make them as effective as a player in Solo Mode.
Players in "Solo Open Mode" will get the same bonus as players in Open Mode but they are as effective as Solo Players. The result is that players in "Solo Open Mode" are now twice as effective as players in Open Mode.
The result is a huge incentive to play in "Solo Open Mode".
Half the amount of players in "Solo Open Mode" are needed to completely nullify all actions of players in Open Mode.
It's often mentioned that most/many play PP in Solo mode because it's the most effective way to play PP. Therefore I assume that the required number of players in "Solo Open Mode" is actually not that high compared to the overall number of PP players as the number of players in Open Mode is low.
It's really a bad idea. Maybe based on a good intention, but the result could be really bad.