They can continue enjoying the game without attacking player faction. There is a reason they are in solo. They are playing for themselves. The only loophole here is the people directly attacking others within it.
I even sent FDEV support a ticket when it started to happen. And they even said it was a new can of worms they didnt foresee. Expect changes.
You are under false premise that opposing someone doing something MUST be PEWPEWPEWPEW! Someone is moving on your faction, do same they do only opposite. Tada! You have thwarted the evil invasion using equal measures.
Same for powerplay and everything else. Do the tasks which strengthen your own position. That is perfectly fine and equal playing ground.
If you don't want to, do not complain that someone might somehow affect your thing when you are not willing to work for your thing.
As has been noted, in thread and to gather it all up:
1) There is no evidence that it is solo/PG who are ONLY against you. Some of them are also FOR you. Thus, they to unknown extent equal out.
2) There is no evidence that solo/PG is THE reason why you cannot see people who affect your situation. Crap connections, denied p2p, different platform, timesone or else different schedule. ALL those affect it. You cannot claim to have 24/7 absolute watch over any given location in all different platforms to be able to say that it is the Evil solo/PG who are causing the change.
Then there is matter of removing content for people. That is very stupid and shortsighted policy. But let's for a moment pretend that Frontier pulls off something and FORCES everyone from solo and PG out into Open by making solo/PG experience much more limited and generally worse. Has anyone thought what is the end result?
I can tell you. It will be for short while all YEEEHAAA!!! RARE AND MEANINGFUL! PEWPEWPEWPEW DAKKADAKKADAKKA. BOOOM BOOM BOOM!
PvP crowd will be ecstatic, FINALLY lots of emergent slaughter, oh sorry content.
But does anyone here think that people pushed from solo/PG by making them incredibly unrewarding experience will just grit their teeth, go to rebuy and rinse and repeat for MO' DAKKA!?
Nope, rapidly there is "So... Blown up again. Anyone remember what that new game that came out/went to sale recently was called?'" And PvE crowd will start to leave the game.
This will be MUCH bigger group that PvP players leaving, because majority of players are NOT PvP-players. (we have evidence of developer having said majority of players are not participating in PvP and when we look at other games trend is that PvE-servers are much more popular than PvP ones, even without looking into possible unbalances on PvP server populations that make it gankfest for some sides and PvE with minute risk of PvP for the other)
When PvE crowd leaves the game, game starts to die. Ultima Online almost died because of unrestricted PvP. They made correction and created pure PvE side and hit golden age of growth.
I came across another MMO in same vein recently. Pathfinder Online. It was all about PvP almost everywhere and anytime. Currently I think the development team was first reduced to 3 guys, and maybe later to 1 guy, who keeps the mostly dead server up and running to some extent for small (and apparently dwindling) number of players.
But it is unlikely it would get to MO' DAKKA-phase. With Elite having rather shallow offerings in terms of things to do, once you start to make solo/PG less entertaining players will not move to open but directly go for second stage of my described process and just leave the game.
Idea that this could be avoided by having players police players is also completely dead in inception. It has not worked ever. It has been tried. Idea of Ultima Online was that players police players. Pathfinder had same idea. Both went total gankfest and neither could get that "player police player"-system to work. Because it cannot work. Amount of disruptive players always outstrips amount of players who want to be police force. Not to mention that police has to be in sufficient numbers in right place in right time to have any effect.
And then they might want to play the game in some other form too.