What is the pH of kvass?
According to this paper$^{[1]}$, the kvas in their study started out at a pH of 6.1 prior to fermentation, then became stable at about a pH of 3.8 after 12 hours.  This is of course significantly less acidic than a cola with a pH of 2.5 (and some colas can have a pH closer to 2).

1) QUALITY CHANGES OF NATURALLY FERMENTED KVASS DURING PRODUCTION STAGES, Department of Food Technology, Faculty of Food Technology, Latvia University of Agriculture, Liela Street2, Jelgava, Latvia