Adding to Michael Borgwardt's answer, there's even an instance of Caesar re-building the village of Asterix and Obelix, so the stalemate between Rome and the Gaul village seems a choice of sorts. You could perhaps say that there's an unspoken non-proliferance treaty in place.
After all, isn't it better for Rome to have one unbeatable village, that wants nothing but to defend itself, than to risk all the barbarians at the border getting hold of the potion recipe?
This is linked to a recurring theme in the Asterix series: The cultural dominance of Rome is even stronger than their military dominance and what Asterix and Obelix does is often asserting their own culture. There's Obelix' disrespect for finer Roman cuisine, patriotism and the communal meal, Asterix journey across "France" to gather objects of cultural significance and their friendship with other european "minorities" which also assert their own identity in defiance of Rome. In the world of the comics, many of the gauls outside the village are "romanized" and do not contemplate an uprising (with or without potion) against the empire they have become a part of.
So from an in-comics persepctive, it's perfectly clear that the potion alone cannot beat the empire and so an all-out offensive from the romans carry little benefit, but with the risk of potion-proliferation.
From an outside perspective, it's pretty clear that Goscinny and Uderzo were referencing, in veiled terms, the cultural dominance of the USA over their native French culture. Complete with loanwords, imported cuisine and foreign luxury goods. If this seems far-fetched, be aware that this is such a central talking point in France and elsewhere in Europe that you can hardly discuss "French culture" for five minutes before referring to the issue of anglo-american cultural dominance.