What the heck does all that have to do with the DF-21D? The Mig-25 was spotted and the US initiated the 4th gen fighter program. The Soviets didn't need to demonstrate its capability to get the West concerned. Same with the DF-21D, as long as the US knows it exists, they will be concerned. China doesn't need to demonstrate the DF-21D's capability to get the US concerned.
Ask the US back in the 70s why they needed the 4th gen fighter programs and they will say "because the USSR has got a new plane", ask them have they seen the plane's capabilities and they will say "no". Ask the US today why they need counter-measures to the DF-21D and they will say "China has a new missile", ask them have they seen the missile's capabilities and they will also say "no". Even if China has not done testing of the missile against moving targets, the US will still try to find methods to counter its supposed capabilities.
It is funny why are you asking me why the heck got to do with DF-21D? It is you who used the Mig-25 example in modern carrier tatics thread. Don't tell me DF-21D does nothing to do with dealing carrier? In fact, I need ask you why would you bring up an irrelevant example of Mig-25 to make your point? Now you twist your words by asking me?
I say again, your example is terrible. Please use a relevant example to DF-21D. DF-21D test is not able to hide from US intelligence. Its either you hit it or dont. DF-21D must have conducted some test which convinced US the threat is real.
And why would US wait until now to have a sample similar to DF-21D that can emulate its trajectory to be build?
The talk about DF-21D is more than 6 years ago. Why wait until now to take such more drastic action?