If the lack of other new players to play with is the #1 factor against keeping new players, that's kind of hard to fix. Unless a critical mass of players joins up all at the same time, they're probably not gonna be able to sustain the fission reaction for very long.
Splitting the tiny trickle of new players into an array of starting cities doesn't help but that is such an old stick that it's hardly worth shaking. Sure they could hop into the portal minigame they know nothing about, but it would be a lot simpler if they didn't have to.
You hit the nail on the head with the 'critical mass'. Part of this issue is that old players don't roll alts continually (like they do on p99.) I think we have too much high end content and that SoD is too mature. There is almost no reason to start new characters from scratch because at 65 the game basically restarts and becomes this extremely long/fun brand new game.
On p99 many many people have multiple level 60 alts. This is because, when you level to 60, max xp, and get those few drops you want there is not much left to do.
-Yes, you can raid sky or clear fear/hate for hours a sitting, but there is a very low limit to this being worth it...
-You can then bat phone raid, which doesn't actually take up that much time and is usually done around leveling alts.
-You can camp your VP key which is about as much time as as maybe 100 SoD AA (if you are very unlucky...)
-You can finish your epic, which has guides and is fairly easy to do at this point (especially if you are in a raiding guild. Many epics can be multi quested...)
-You can farm cash/cash drops to gear alts.
I just summed up the entire p99 end game in less points than it takes SoD to get through tiers that are worse than BoE. If you want to keep playing in p99, you need to roll alts. Vet players are cycled though with new players and everyone is LFG together. Vet players in end game gear are not so horribly far off from fresh 60s that they can't group together (as they are in SoD.)
Again, SoD has a huge 'end game' with mega long quests and many many tiers of bosses. Vets typically don't start over at level 1 because of the time investment, and because of this new guys don't get the help they need from the vets. It is kind of a vicious cycle where the end game gets heavy and the low level game gets empty. I think we are at the logical conclusion of this cycle where very few people want to start a game with zero population for 65 levels (and this conclusion is hurting the end game because there is no new infusion of fresh blood.)