The problem with the Forsaken is they can only repopulate by successfully conquering an area with suitable bodies to revive. This means they have to go on the offensive, even at a disadvantage, to have any sort of chance. The Forsaken's numbers are dwindling over time after the immense casualties they suffered in Northrend. Now that they've lost more in skirmishes against the Worgen and more in Siege of Orgrimmar, the Forsaken won't have the manpower to be able to try their hand at conquering.
The only reason Gilneas was remotely successful was because of the timing. The Gilneans had just lost their city to the Worgen, lost part of their land to the Cataclysm, were trying to regroup and tend to the survivors, then found themselves under siege with biological weapons. And they were still able to beat them back.