Well, the interesting thing about this justification for storing metadata is that if you accept it, there's absolutely no reason it shouldn't apply to contents too. If you're allowed to store data indefinitely because only
access to the data should be controlled, and you trust politicians and intelligence services not to abuse that power, why not do the same with all data? If you have control over access and the data will only be used in approved ways, why not just store everything and decide later what you can access?
Metadata is just as dangerous as content, and frankly I think the only reason content hasn't been stored is that it is a difficult technical challenge, and they don't yet have the capacity. Indeed, we've seen proof that the UK is already doing this for as much data as they can manage, without the permission of any of the US citizens whose data they collect wholesale in transit and send back to the NSA.
The massive danger here, which Snowden pointed out, is that if you store this information, policies can easily change in future to allow access (as they have done already with metadata), and at that point nothing can be done, because the information is already available, just waiting to be used, and going back decades. By allowing collection, you're trusting not just the current politicians, but every conceivable politician and intelligence agency over the next few decades or centuries, and all the foreign intelligence agencies and politicians that the information is shared with. These policies are of course secret, so we'd have no idea if they have changed or what they have started to use.
You also have an exponential growth of classified information which you have to try to keep protected, even as you expand the number of people managing it into the hundreds of thousands to deal with the ever-increasing technical demands of storing and cataloguing it. That's going to lead to leaks to the public and also to foreign agencies, and if the ambition is to store all signals eventually we'd get to a point where no-one has any privacy as it is impossible to keep a store of that size secret and contained.
The only answer I personally find acceptable and practical is to forbid storage of the information in the first place, restrict spying to clearly defined targets with proper judicial oversight of individual cases, and limit the time it is stored.