My main concern is that should "Open PvE" appear on the same list as the other primary modes, many people would flock to it without even giving "Open" a chance. This would fragment the player base further, and probably not in Open's favor. Many people here won't care about that, but I certainly would, and the FDev has made it clear that their vision involves potentially hostile, organic, and spontaneous, interactions between CMDRs.
A secondary worry about such a mode is that it would make it easier to organize manipulation of the background sim in a largely risk free environment. All the reasons given about why private groups are less than ideal solutions to the PvE problem are the same reasons they are less ideal for playing the BGS.
To your three essential sub-points in reply -->
1- many people would flock to open pve mode if appears on main game mode start list
Seems you're admitting the self justification of why open pve should be on list then. If there is enough who prefer that to cause a flock to it, then as surmised by many who advocate for it, pve is indeed the substantial majority.
I do agree with you though that this would cause fragmentation of current open. But if you can admit that people would flock to open pve, then it would be more correct to say regular open is fragmenting the majority open pve mode, not the open pve mode fragmenting the regular open mode.
2- FD vision re potential hostile interaction between players
I agree, but the operative word to stress is potential, as in not mandatory. The only 'will happen' hostile interaction is npc / pve, although I suppose it is remotely possible if you explore long enough you could mostly avoid that.
Potential pvp only happens if you join open, which by definition means FD clearly provided solo and private groups as means to completely and 100% avoid hostile player interaction.
*before someone says moebius can be infiltrated and pvp attacked, that's why I said private, not moebius. When I play in my private family group of brother, cousins, and nephews, there is zero chance of hostile pvp. Same for solo. Bottom line is FD provides for potential hostile player interaction but also designed guaranteed methods to avoid it as well.
The sole difference in above and what people are asking for in open pve is the admin would be FD, not a private player (who pays multiple accounts, admin time, etc), and joining would be automatic rather than manual per today's private group.
3- risk free BGS Impact via open pve mode
I presume you mean risk free from pvp interaction, because the same pve risks would exist. This isn't a BGS pros/cons thread so I won't go into what should be the BGS mechanics and just go with what we have today.
Which means, so what if people affect the BGS in open pve? It would be the same as ability to impact BGS playing solo, completely risk free from pvp interaction as well. And moebius, and any private group of friends, family, coworkers, etc. They all allow BGS impact without pvp. So an open pvp would be no different.
There a lots of valid points people have made in other threads why BGS impacts should be different in open vs solo modes, but I'll leave that to those threads as off topic here. Point is that based on what we have today, your concern here re open pve impact on BGS is a non issue.