It's way too easy to exploit "open only BGS".
Say Frontier did make it so that only actions taking place in Open counted for the BGS. If I buy some trade goods in Open, then switch to Solo, fly the trade goods to the destination station, switch to Open, and sell the trade goods, then all the BGS-relevant actions around those trade goods have taken place in Open.
Or let's say I go exploring in PG and bring back a load of exploration data which I exchange for cash. I then dock at one of your stations in Open and use that cash to complete hundreds of donation missions.
Or lets say I take a mission to get 50 Palladium for your opposing faction. In Open, where the BGS is active, Palladium will be very hard to find because the market restock rate is low and it will mostly already have been bought up. In Solo, buying goods doesn't reduce the amount available on the market, so I can just buy it all up easily.
The BGS is embedded throughout basically everything we do, to the extent that "Open only BGS" means either
- it's so full of exploitable loopholes that it's basically what we have now but you get to see a bunch of people relogging in your stations
- Open is the only mode at all
- Open and PG+Solo have entirely separate galaxy simulations (including markets, first discovery lists, station services [1], system names [2]) and you choose which one a CMDR is in on account creation
None of these are going to happen.
It's not like Powerplay which could in theory be isolated to a single mode without affecting much else.
[1] CGs could easily have different results in one than the other, for example
[2] Part of the prize for player groups winning the Colonia Expansion Initiative CGs was to be able to name their own starport and system.