You did not buy a guaranteed life-long access to online Rocket League in all its future forms.

Your purchase of Rocket League came with access to the online service as it currently stands as a trust only relationship between you and the owners of the online Rocket League service.

I understand your pain here, and I don't want to defend Epic games, but as someone who has created multiple free cloud services only to have their costs or development requirements exceed my ability to maintain them I am weary of anything that would legally require me to maintain a cloud service past my desire to do so.