A replacement admin should come from the gmod realm, preferably the most active one. Unfortunately, I can't say with full confidence that Ex (the most active one) would be the best suited as other mods have been known to disagree with his moderations. However, giving Ex the power he has now and used to have certainly was a learning experience for him, and he is now a better member/gmod for it. I'm confident the same would happen if DeV were to be given admin rights. If it's clear he should not be in that position, then he will be demoted as Ex was.
If DeV isn't promoted to admin in the near future, he will never be, as I'm afraid SEN will either a) die, b) I will leave (see a), or c) DeV will leave. IRL happens, you know? On top of that, when will he be ready? I was still an idiot at the age of 19, but that's just me.
Of the gmods:
Devlin - I forever thought he was a gmod/used to be one before he was one. If he wants it.
Farty1billion - If he wants it.
JaFF - this impariality is new to me.
Excalibur - Guaranteed change, possibly losing many vets after getting tired of him, but has superb activity.
Merell - This would be the ideal choice, but he's probably ready to retire too/will take too many breaks.
I think with more responsibility you learn faster. Anyone could be a suitable admin as long as they want it (so that they would be active) and have some form of moderation experience.
Really moose's admin job is judgement calls when/if there is a moderation dispute (please correct me/add on if I'm wrong or there's significantly more). Do we really need another moose admin? No. A DeV admin wouldn't make the same decisions as Moose, and more than likely the moderators would take a vote on disputes. DeV essentially IS an admin right now, just without the title/respect that goes along with it. SEN is an oligarchy mixed with some democracy, and will forever be that way. If we lose some of the "rulers" the site will still function, and eventually new leaders will develop.
"Parliamentary inquiry, Mr. Chairman - do we have to call the Gentleman a gentleman if he's not one?"