No. Novgorod had to import the majority of their food from places further south. It thus had a critical weakness which can (and OTL was) be exploited. The system itself wasn't the most stable, either. For Novgorod to unite Russia, it will need to invite a prince and effectively become a monarchy.
But Republican Russia is not impossible--you just need Russian politics to evolve in a direction similar to Poland-Lithuania (remember, Poland-Lithuania was considered a republic and all Polish states since 1918 use the same word "Rzeczpospolita" for republic as Poland-Lithuania did). That is what many boyars and other nobles had always wanted, and was probably doable at any point before Peter the Great and especially without Ivan the Terrible doing his best to break noble power. Poland-Lithuania is incidentally what a successful Novgorod or Novgorod-united Russia would probably look like.