I would love to see relegation come back... one day. It's a cultural thing in this part of the world. It's great to watch the excitement of getting promotion or avoiding relegation.
The main issues are:
Clubs in the Championship aren't of standard yet, so won't integrate into SL very well if promoted (and will end up being most likely relegated again).
Clubs in the SL aren't of the standard wanted yet either, and we would like to see them at a standard wanted before being able to say that Championship clubs aren't of standard (so it isn't hypocritical).
Relegation is not practical for certain expansion clubs (i.e. French clubs), so we need to find a system that fits in and is fair.
The first two points should hopefully only be a matter of time before the licensing system brings them up to standard. SL will in the next 3-6 years have clubs with good structures and good facilities that reflect the game. Hull, Wigan, Huddersfield, Warrington and Harlequins all have excellent and modern stadia. Crusaders are only one stand off. Saints will do soon. Catalans are in the process of building. Leeds and Hull KR have plans in place. Salford, Cas, Wakey and Bradford need to get a move on and catch up. In the Championship, Widnes have the package. Halifax and Leigh are there. Toulouse have a ground in place if they get promoted. Fev have plans. So we're getting there over the two divisions.
The issues with relegation need to promote expansion as well as tradition. How do you get a fair system that can do that without "putting French/overseas teams in the Championship" which isn't exactly financially viable?