This is a really good point and my 1st instinct was to agree totally. Nowadays, fighters are supposed to be fitter, there is more hi-tech nutrition and conditioning, more stringent medical rules, so why not have 15 rounders?
But......... The negative side is that a lot of fighters would conserve energy to make sure they last the distance(they already do with 12 rounds). This would mean the early / mid rounds could be a total bore. With that in mind, there could be an argument to reduce it to 10 rounds , so that fighters wet he'll for leather at it.