Enforcibility is not the point, its the threat
Whether the law is enforcible or not is not as relevant as the fact that once this law is in place your average joe (or pierre) is going to download less. Look at the example of Sweden:
"Swedish Internet traffic measurement firm says it fell from almost 200Gbs to 110Gbs on the day the new law came into effect.
Swedes are apparently scaling back their Internet usage after that country’s controversial new copyright law went into effect last Wednesday.
The new law, if you recall, allows copyright holders to seek a court order requiring ISPs to divulge the names of accused file-sharers.
After going into effect, Netnod Internet Exchange, an Internet traffic measurement firm, reported that Internet traffic in that country by almost one half, dropping from almost 200Gbps to 110Gbps on the day the new law came into effect."
Obviously thousands of people stopped using p2p at the time. Would they all have been caught and taken through the legal system. Almost certainly not but it still was enough of a deterrent to put them off. Any success of the french law should be measured not by how many people get warned etc but what effect it will have on the general level of p2p traffic in france.