Once upon a time, it was really very easy to figure out which martial discipline was better, you simply looked to the battle field. If the practitioners of one style tended to die a lot, then that probably wasn't the best way to go about. The ones who walked away alive, however, must have been doing something right.
Even then, without the benefit of any battles being held conveniently nearby, you could always send your best students over to the other guy's school and have the compete against their best guys and whoever won, maintained the bragging rights. I believe Kano once put his boys up against the Tokyo police and won quite handily (and I believe judo is now standard training for the police).
But now, well, now you have arts like jodo, iaido and whatnot (the kyuryu, old school arts), who don't have the opportunity to use what they know in any sort of real, practical, battlefield application, and spend much if not all of their time in kata. I imagine that they assume that their techniques were once successful on the battlefield, hundreds of years ago, and must still be just as practical. But no two people do things the same way. Variation inevitably creeps in, and reasons get lost.
(Of course, some arts, like judo or kendo, have made a sport out of the art, which helps somewhat in terms of "if it doesn't work, throw it out", but that can also turn into a matter of someone getting really good at playing a game, with lots of rules and limitations.)
By way of example, there's the story of the woman who wondered why she always cut the ends off of her roast before cooking it. When she asked her mother, who taught her how to cook, then in turn her grandmother, she finally found out that it was because her great grandmother didn't have a big enough pan. If I remember correctly, even Pascal Krieger, in his seminal book, Jodo: The Way of the Stick, admitted to not knowing the reason behind one particular movement in the kata.
And where there is no randori, no shiai, no battlefield testing, it seems like the default means of arguing the truthfulness or correctness of a school is to rely on it's lineage. If your teacher was taught by the right teacher who was taught by the right teacher, etc., then you're the real deal and anyone else is a poor imitation and a hoax.
I'm sure there are folks who disdain aikido, being a gendai art, but I've also seen aikido practitioners turn around and similarly dismiss the Tomiki ryu. Unless your teacher is connected somehow with O-sensei, then by golly, you're just not legit.
Sure, to some extent, a person's lineage also serves to ensure the new, perspective student that their teacher isn't just a charlatan after their money. It's only when the need to cling to tradition and lineage clip the wings of a genuinely bright and innovative practitioner of an art.
All of that being said, there's probably a great deal of truth in the koryu systems and schools, but what do you do when they turn up their noses and casually dismiss what you have to offer?
Can we not share? Can we not be open? Is true budo not supposed to be love? Is it not true what my fellow budoka John Winter said:
"'True' aikido, judo or jodo or whatever one endeavors isn't in the art or ryuha, it's in the practitioner. 'True' anything is the heart of the person following the path."?