Does this mean that languages become more synthetic if they're left on their own? Did Latin and Sanskrit develop in greater isolation? This is a fascinating subject.
Within the Indo-European family that includes Latin and Sanskrit, the empirical historical process seems to have been that the Proto-Indo-European language was highly synthetic, and as time went on, less synthetic daughter languages such as Latin and Sanskrit developed from it by degrees. These in turn gradually developed by degrees into even less synthetic / more analytic daughter languages (Italian, Hindi, etc.)
Why this happened is an open theoretical question. Pidginization (which would roughly correspond to 'lack of isolation'), and/or phonetic changes that impacted discernment of synthetic features are leading candidates.