Take note: Tiziano Motti
Oh it sounds so measured, so reasonable. But it does mean investigating every last one of us to pick off the few bad apples, maybe. IIRC not the first time he's proposed another "think of the children" canard. Soon he'll have earned the "paver of the road to hell" merit badge.
Data retention has already been shown to not significantly help. It does promote taking dragnets to datastores, and that does indeed increase, sometimes lots. To me that indicates it promotes lazyness and facilitates arsecovering in a perverse sort of way.
There's so many better things you could be doing. You could be putting better effort into catching perps in the act, whatever they might be doing --if you need to single out notorious, uncommon and narrow hot buttons for such broad and general measures your arguments aren't very strong--, and for that you only need to be able to legally, with warrant in hand, tap any and all thing they might be using to communicate.
We're not trying to do extensive pro-active pre-suspicion traffic analysis spookery to the populace, are we? Or were you thinking more of lucrative side uses of all those black boxes and easily mislaid keys, don motti?