Mark Hamill as an aged Luke: That's like the best nostalgia drawcard you could come up with. All the old farts who haven't paid attention to anything SW since the OT will go along, just to relive a bit. They might not show up to something they could write off as post OT silliness with Mouse ears. But give them a trailer with the elder Luke staring at Tatooine's suns setting and it's gold.
This will have absolutely zero effect on a young audience who grew up on the PT. They will be there.
This. x 2.
x3 ... don't discount nostalgia ... and don't discount the need to establish a new identity. A "passing of the baton" scenario is highly likely - and will no doubt result in endless internet nerdrage.