I love threads like this. And it gets better ..... I'm not in favor of any since I believe both are sensible ... from their own stand point.
Of all bettas, splendens are the most aggressive. But in the wild they are not as aggressive as the domesticated ones. This is because no one separates older/larger from younger/smaller (IMO not because they were selectively bred to be aggressive). What ever animal will have poor mentality if they were intimidated since young.
Yes people use "winners" as breeders. But if the offspring were left with the adult, they will become "losers". So attitude is "planted" by isolating each male since a very young age, building up his self esteem to the maximum. And "death fights" are IMO staged by using rather deep containers. Otherwise the loser would only run.
So, IMO, it's possible to create docile splendens. And these would still show glamorous colors. You don't have to introduce other species to achieve this. But then again, do we want them to be as such? Without serious fraring, they don't spread their fins to the fullest and there fore lose their form beauty. Further more, most of the time, these docile males will at some point flare and bite thus may have torn fins. So it all depends on personal preference.