This is indeed a threat, and I don't assume China can't resolve the issues of finding and targeting our carriers far from China. Nor do I assume our missile defenses could shoot them down. So they are a threat--as any land-based air power is a threat to our fleet. But this is not revolutionary unless the Chinese build intercontinental carrier killers capable of reaching our West Coast to do a Pearl Harbor on us all the way here. (Ahem, BMD anyone? They're not just for rogue nuclear attack defense.)
So the result of both the DF-21 and other shore based assets like planes and cruise missiles is that our fleet has to be far more cautious in approaching Taiwan to fight the Chinese and help stop an invasion and ship in critical weaopons and ammunition so the Taiwanese can fight. such caution could be the difference between China winning or Taiwan holding out.
But we are not helpless to hit the Chinese invasion fleet even if our fleet must clear the way before we dare risk our carriers in close. One, our forces on Guam and Okinawa can help. Two, our subs can move in. Three, smaller missile-armed warships can head for Taiwan and China may not wish to try out their DF-21s on such ships to avoid tipping us off as to their effectiveness (or lack of effectiveness)--or it might be thought a waste on smaller ships.
Another way to fight the Chinese who use land-based long range weapons to keep us at a distance is to return the favor (from my Jane's email updates):
DARPA progresses Long Range Anti-Ship Missile project
Two separate project teams within Lockheed Martin Missiles and Fire Control have completed initial Phase 1 design and demonstration activities under the Defense Advanced Research Projects Agency's (DARPA's) next-generation Long Range Anti-Ship Missile (LRASM) programme. The rival concepts - LRASM-A is a low, slow and stealthy missile, while LRASM-B is a high, fast and manoeuvrable solution - completed their respective preliminary design reviews in March 2010 following completion of near parallel nine-month Phase 1 contracts awarded in mid-2009[.]
We can match China in range of anti-ship weapons. We could strike the Chinese invasion force from a distance even without using planes to carry such weapons within range if the weapons match the range of the DF-21.
This doesn't help with the issue of resupplying Taiwan during a war, of course. If Taiwan would build up and secure war stocks of ammunition and other supplies, they'd buy us time to pierce the shield that China is trying to build east of Taiwan.
