The limited numbers of those missiles is another thing. You may think that 500 or 700 is a huge number, but in reality its not so impressive if you use conventional warheads.
Without taking accuracy into question it is actually quite a concentration of firepower. A single DF-11 has warhead of 500kg's, sure, but taking kinetical energy into account it's probably worth about 1000kg, or 2000lbs bomb.
Let's compare this to an airstrike, let's start with 500 missiles. To get similar shock effect of 500 2000lbs bombs hitting the ground simultaneously you need 500 airplanes alone if your aimpoints are not within convenient range. But let's simplify this and say each of the strike aircraft will have two bombs. That's 250 strike planes then. But, to get those strike planes through a number of SEAD sorties, fighter sweeps etc. have to be flown.
Another issue is PGM's. Yes, they're more expensive but they are extremely cost effective as less bombs have to be delivered than with conventional bombs.
This topic is old, so I don't know if this is much of an input, but here's my secret master plan to eliminate Taiwanese air defense. It's composed of various actions:
On day one,
First one is a total aerial effort approaching Taiwan. As PLAAF planes get closer and ROC interceptors are being launched, a massive SSM strike will be launched on airfields, using cluster munitions to catch as much planes on the ground as possible. Known SAM sites will be attacked too.
The first air effort will be mostly a feint, to lure ROC forces into air superiority fight and SAM traps. As ROC fighters endurance gets lower, a second wave of SSM's will be launched with runway cratering munitions. (In reality, this would be almost impossible to synchronize, but SSM's launched together with air effort would ensure that main ROC defense effort would be concentrated on shooting down TBM's instead of PLAAF planes.). A number of SSM's will be used as a reserve to attack SAM sites whose location will be known via EW measures.
On day two,
Repeat of the same, but this time with tactical strike planes actually making to strike the targets. Repeat as many times necessary or as many times as it's possible. Some SSM's should be kept in reserve, as with possible invasion some SAM's will be kept in reserve. SSM's should be used to eliminate SAM's.