The new generation will like OT just as much as the new trilogy. Part of the story, they will watch them.
Parents are more likely to buy stuff/pay more if they recognise it. Emotional attachment is important.
Regarding UCS sets we won't see any from the new movies in the first few years. LEGO will evaluate their success first. Until then old vehicles are much less risk and still sell just as well or even more.
The new movies will get a balanced share from the play sets aimed at children, but because of aforementioned reasons OT sets won't disappear. At least we will be able to always count on the iconic stuff (MF, X-Wing, AT-AT, ISD, etc.) if not on the more obscure ones (B-Wing).
What you must not forget is that LEGO will always exploit lingering demand. They are monitoring websites, blogs and forums. They are responding to that demand. If anybody thinks that the recent UCS-s in flashy boxes are the Sandcrawler and Slave I by coincidence is a fool. Rumors about the next big UCS being an AT-AT. They know that they can't release them as IDEAS, but that project signals demand and they just release them as regular sets. This shows that they are responding. So even if there will be some small draught of OT sets in the future people will surely complain about it and LEGO will just release some MF or whatever to cash in.
If I want to play the role of the Prophet of Doom then my forecast is that the EP I-III stuff will suffer only significantly, but not to complete disappearance either. It is considered inferior, little to no emotional attachment and just simply uncool. Key parts will return from time to time though, like Darth Maul for instance.