I think Entrenchment is the big chance if IC and the community play it right. Sins got excellent reviews, it already sold a big number of copies, and apparently it's the IC/Stardock line to keep their games alive for YEARS with expansions, patches and the like.
And maybe that strategy works. There is always a little rush of new/old players when a new expansion comes out. I dunno if there are now more people playing because of the holidays (I am) but comparing with the numbers Ke5strel quotes, I'm actually seeing slightly larger counts of people online. So that's very positive in the sense that even with all the past problems, Sins online is not going anywhere - the MP in most games just dies well before they reach 1 year, not to mention the space 4x genre where it might never get even started! 
There is a huge pool of potential MPers out there, and IC/SD could capitalize on it by promoting the MP side more, especially when they already have the media attention with the mini-expansion. Some care should be taken to make the MP more accomodating, eg. make sure there are games available that don't take hours and hours, since people have less time available in MP, they can't just save and load later. Players could easily go the route SP-> co-op online -> MP -> clanned MP, if the MP side is promoted and clans/community take care to integrate any new players. Clans and MP is what keeps many games alive, and wins publicity & mindshare for the developer - this is an opportunity for IC/SD!
Some small developers are already putting a LOT of effort into making custom maps official, take a look at World in Conflict for example, they are doing a splendid job there! And yeah, the thing with automatically downloaded custom maps/mods is usually that they are several megs, so who organizes the d/l servers and bandwith, plus it takes time... But Sins maps are like <10 kB each, WTH IC?