Double EXP?

[Niwashi]

I'm just wondering how many people will be willing to do old 8 man content when the new exp is out?

There will be new people, people with new characters, and people who aren't getting the expansion. Plus SE has a pretty good track record for making sure that people have multiple incentives to go back to earlier content if other newer players are going to need them there. That first couple weeks after the expansion launches there may be a lack of people in some of the ARR content, while top-level existing players are only interested in the new stuff and newer players haven't reached the late ARR stuff, but I wouldn't anticipate it being much of an ongoing problem. If it is, SE will just add more incentives to the neglected areas until things even out.