I think Australia's ACCC has established that if access to the product is revoked at a later date, the seller must refund those who purchased the product. In addition, the ACCC established that digital games are goods. So, Australia would be a good place to put forth a case in court. I would contact the ACCC, or somebody in Australia who can work with them. If Ubisoft were forced to refund all purchasers of The Crew in Australia, it could send a shockwave warning to publishers everywhere to not tie offline gameplay to online services.
And if successful in Australia, I'd then look to file the same case in EU countries. It could be possible to file it in the Court of Justice of the EU, whose rulings apply to all EU member states. The process to having a case heard by them should be looked into. It might require first filing in a state court and then being appealed to the CJEU, or it might just require being able to explain why the issue is relevant and important to the EU as a whole.