Obviously, if the material and teaching sucked, or if cheating was rampant, that would tarnish the reputation, but the same could be said of virtually any educational institution.
There are potential ways of addressing this, but they mainly involve the online university developing selection criteria just like elite colleges (e.g. everyone can access basic materials, but only certain X performers can access the premium version with company interviews; or they only admit X students per class). Otherwise the value of the degree is diluted and it has no more signaling power to employers. In addition, the online universities will likely attract less capable students, because for very strong students, top tier universities are still very much worth it.