You are literally forgetting how Sanskrit works. There has always been an unbroken line of scholars who have learned the language and have a vast understanding of the inflection and verbal varieties.

There is plenty supporting evidence of how vedas being recited in vedic schools with aid of oral traditions, are reciting in the exact inflection and speech variation as the ancient times. The oral traditions have literally constructed mechanisms to ensure this as a system that is widely studied as well. Sanskrit is not some language that people stopped using it for hundreds of years. Never the case. Infact the last Sanskrit scholar who wrote extensively in the language was no more than a 150 years ago.

There have been several Sanskrit schools of learning before and after that person. You are in denial of how the language actually functions and exists and studied continually. And all of these do cause language variations and trends just as much as any other language, or maybe fewer, but not zero.