My red and yellow carts still have original batteries too, and the saved games still work perfectly (as of two years ago when I last checked). The only thing I might say "oh well" about losing is the official Mew I got from some Pokemon event at a mall, but I don't even have the certificate that it came with so it doesn't really matter.

Gen II (Silver/Gold/Crystal) had a time-of-day thing going, which I expect probably required more battery power due to also needing to maintain a clock along with the save data. Gen I only needed to hold the save data. That would be my guess for the difference in battery life.