Full backs seem now to either be very good offensively, or very good defensively. That seems to be the way it is now, in the same we now have "defensive" or "attacking" central midfielders when we used to just have central midfielders.
Unless we're going to go and spend £40 million and buy Phillip Lahm (who doesnt actually seem to even play full back a lot of the time) then we're looking at a limited pool of players. I think to do well at full back in the PL you need certain traits that might not suit a lot of continental wingbacks. The last think we need is player who's more interested in getting forward than defending. You need to be quick and can get away (potentially) with not being that technical.
I can't see that there's a great deal of players out there who'll fit that mould. Alves perhaps, but he's the sort of player who could look his age very quickly in the rough and tumble of the PL. I don't see the sense in dishing out long term contracts to 32 year olds who've been moved on from other top sides seemingly because they're on the way down.
On that basis, signing a young English lad with a lot of potential seems a solid and sensible decision.