This validates all extremism (B) by claiming that it must have had a valid complaint (A).
Not that it must but that it does. That doesn't validate anything, it addresses reality rather than ignoring it.
What violent extremist movement exists today that does not have a public, democratic, social, economic or otherwise legitimate way of addressing its concerns?
Given al Qaeda's foundation in wahabism, centered in the Arabian peninsula and expanding into south-central Asia and northern Africa, the dearth of democratic institutions is pretty relevant. For that matter, the fact that no targets of US foreign policy have any input into the policy underscores that. This is still not to justify the extremism, but rather to point out that other forms of input are lacking, broken, or compromised.
That said, I'll reiterate that the concerns are worth addressing on their own merits and that it's a positive side-effect that it would undermine the support climate for the violent extremist reactions.