What happens then when the governments of the world say, enough is enough, and then start targetting Bitcoin in order to stamp out SR?
Here is one example: they could start monitoring the exchanges (which they already do to some extent) and then start getting search warrants, phone taps etc for anyone buying or sell large amounts of BTC. There are roughly 100 people involved in the running of SR, all of which are paid in BTC, if just one gets caught who knows what kind of information they might be able to hand over for a deal with prosecuters.
The more drugs in the mail, the more anonymous your package is.
More drugs in the mail will just mean that customs will be toughened up. Most SR sellers have stopped shipping to Nordic countries as some Nordic countries now have what SR sellers call "weed detect" as 99.5% of their orders were being seized because of new technology that customs were using. They've tried everything to beat it, and nobody has managed to yet. Once the volume of drugs being smuggled in increases, every country will have this.