frankly speaking, both the EW and NS line already at its capacity...they cannot add any more trains liao, or increase any more frequency before the trains start to kiss each other's backside.
The only way visible is to increase the travelling speed of each train - but this is limited i think by the infrastructure - the signalling system cannot tahan higher speed without compromising safety. Morever, after the Clementi incident, I think they are even more adverse to this. So, for EW and NS which remains the core transport line - this is it...until the line can be extended sufficiently to accomodate another train.
I think during the setting up of the system, they grossly under-estimated the capacity or they felt it was not cost effective to factor in the high usage.
Basically, the length of the line limits the number of trains possible, of course, increasing the travelling speed of the train helps but there is safety concern. But increasing the frequency of the NS trains only solves the problem partially. You bring more people from Sengkang to Dhoby Guat - they will jam pack the trains to Raffles Place and City Hall where most of the crowd are heading. So unless, they extend the line to the CBD, they are not doing anything constructive.
The most practical thing to do, is to increase the speed of the trains and not keep making useless statements of increasing frequency on NS line which adds more people to the fringes of the city. Or what they do with Circle Line, shorten the train to allow more trains giving the false impression that the trains can come faster.