I feel that I have to put up the disclaimers first:
#1: I'm a theorist. Think of everything below strictly as theory.
#2: I don't live in the US - so I see things with foreigner's eyes. If I'm missing something or there's something I misunderstand, then it's not intentional.
With that being said...
Obtaining the necessary warrant might prove to be impossible without obtaining communication-based proof beforehand. Today, they see that 555-0101 called some number from Afghanistan 2-3 times a week during the last 6 months, and some of them came from chemical products shops, while others came from hardware stores, electronic pieces stores and the White House (during tourist visit hours) - so there are some flags raised. If they can't do that anymore, they won't know and won't be able to take the appropriate step to prevent something truly horrible from happening. Then everyone will yell that they didn't do their jobs.
I can't say I agree with NSA's current methods, not at all. But at the same time I can't figure a better way to prevent impactful, unlawful acts from happening (from terrorism to major drug smuggling and so on and so forth). In this specific area I kind of agree with him. Is there a better way? And by "better" I don't necessarily mean "let's go full legal and there you have it" - that's probably way worse from an outcome perspective. What if (again, as I said above, theorizing here) the NSA stops collecting that data and within 3 years the amount of bombs going off increases tenfold, while at the same time drug usage increases by millions of souls, meat trafficking gets out of control, etc.? Then it will be widely regarded as being "the worst decision that could possibly be made".
The higher national security, the greater the costs (and sacrifices of personal privacy). It's valid for pretty much every country in the world. A balance must be stricken, but weights on both platters are variable and subjective.
I know that right now everyone and their dog is up in arms about NSA spying on citizens, which shouldn't happen, but at the same time I can't imagine any other way to tell who's the innocent citizen and who's the evil plotter who wants to blow everyone up. My trust isn't with the NSA but at the same time isn't with the Average Joe saying "you should stop it, period", either.