Crota can be solo,d..... Atheon cannot. Enough said. The fact is this no matter what ground you are on. Atheon can control time. Crota would be lost in time before he could chop enough damage without times vengeance before teleported into some random corner of time. Not being able to return crota would be lost. Atheon is the true god. Time is the ultimate weapon. The vault is nothing more than a Throne and a resting place there is nothing that suggests atheon was even inside then vault before his time of awakening. Crotas layer is nothing more than a pit for a goofy green giant and a gooey over soul.