If FDEV goes with that poor decision, technical measures will be taken to ensure solo play in open. That should probably do the trick.
If that doesn't help, a refund may be in order.
Online games change all the time, and every single game developer has the right to do that.
First: E

was not sold as online game. It became online only after FDEV could not get the backend coded into the game and put it on separate servers.
Second: Yes, online games change, but usually features are added, not removed.
If I buy a license for a game, I am usually allowed to use it indefinitely.
If every gamer around the globe who didn't like their game changing or getting updated, and refunds were the norm, there would be no game developers around because it just wouldn't be viable to keep anything afloat, it would just stagnate and die.
Do I look like I care?
Developers can either stick to what they have sold, sell upgrades and earn money, or they can remove features, destroy the playerbase and hand out refunds. If it doesn't keep them afloat, it's their own fault and they shouldn't survive.
As a gamer, if I didn't like an upgrade, I simply don't install it. Or buy the expansion. That's the way it has been for decades.
Any game that is online, and needs any type of online mode to survive (Private/Open), will get updated - any game improving itself through content or patches goes hand in hand with anything online.
See above: The game being online is a workaround in itself. It is therefore FDEVs responsibility to make sure any changes are compatible with people's gameplay. We are not talking about games like World of Warships were things are supposed to change all the time.