What is there on the page says that Encounters this season will be limited to the two Essentials books. It implies that future seasons may be limited as well, but at no points says that those limits will always be Essentials and Newer content only.
Could you be right? Maybe. But that is cerainly not what they are saying right now, and interpreting what they are saying as, "Pre-Essentials material will never again show up in D&D Encounters" is absolutely reading something into it that isn't there.
Feel free to state that this season is Essentials only (as that is what they have said.) Feel free to speculate that future seasons may never incorporate any other series or books except newly released ones (you could well be right). But I think it is premature to claim authoritatively that this is stating that "moving forward no PHB1,2 or 3 characters. Not for a season but for good."