I wouldn’t expect ERA to recover when TBC goes live, I would actually expect a bump in P3 of Fresh, then a drop again when TBC goes live, then assuming TBC becomes an ERA server, for ERA to pick back up as Sunwell launches, where you start to see populations balancing on both servers.
This will make it tough for things like BGs in ERA, but world PvP should be fine if a bit more limited, as well as most PvE raids should still be ok since they are manageable with smaller populations.