Well, this is a very interesting question. I've wondered on this before because such a rigorous preservation of a number system as we find it in the Indo-European languages is by no means the norm. It's for example a great difficulty in the Turkic languages, and as I mentioned, there is the hypothesis (under the assumption that the two languages are not closely related) that either Basque or Iberian might have completely borrowed the numeral system of the other (I must admit that I am rather sceptical of this, however). On the other hand, it is definitely interesting to look where the part of the number system were not preserved.
One (semi-)example I would like to bring up would be loan-translations from Etruscan into Latin:
18 - 'Duodeviginti' ('two away from twenty') - Etruscan 'Eslem Zathrum'
19 - 'Undeviginti' ('one away from twenty') - Etruscan 'Thunem Zathrum'
The interesting (and amusing) part is that the Romance languages have reversed this Etruscanism (sucha s French 'dix-huit', Spanish 'dieciocho'). However, this is also not really an example of a divergence from the PIE numeral system since the IE root words are perserved in the loan translations anyways.
Regarding a reason for the overall conservativism, the pastoralist explanation certainly seems like very reasonable explanation at first glance.