One future tense for every verb: what Serbian built that Russian never needed to
Neither Serbian nor Russian kept a single inherited future-tense form from Common Slavic - both had to solve the same missing piece some other way. Serbian's solution is one auxiliary that works on every verb regardless of aspect. Russian's solution is not a solution at all in the same sense: it split the job in two and let the aspect system itself do most of the work. A Russian speaker's instinct to let aspect decide the construction is exactly right at home and exactly wrong in Serbian.