If Ueshiba's disciples are acknowledging that he was a poor teacher, why not be more open to altering the system instead of doggedly adhering to his method of teaching?
Big question. I think that a lot of factors come into play. First, excessive respect towards the "creation" of Ueshiba (although almost 100% of what he did was Daito Ryu, except he expressed the concepts in terms of his Oomoto-kyo religion rather than the original Taoist-derived stuff). Second, an ostensibly strong focus on the "spiritual aspects" of the art, despite a lack of comprehension of these aspects. Third, a lack of technical leadership, as the biggest names in aikido have always had diverging methodologies and ideas. Fourth, the ego of masters that prevents them from questioning their own practice. Fifth, and most importantly, a lack of a common goal. Every aikidoka trains for different purposes and the only thing that kind of brings us together are lineage and a few common techniques (that everyone does differently anyway). You can't have any quality control when there's no quality standard.
Unfortunately, I don't know many in the Aikido world who admit or understand this, though it does appear to be the case. I suspect he seemed to be teaching well to some (they thought they were picking up what he intended), but core information was lost. Or he left it out, which would be about as bad, from what I understand of the base of Daito-ryu.
I think some folks picked up some of the key principles, but my knowledge is too thin to understand whether what those folks passed on includes the core of Daito-ryu.
Well, nobody knows what was going through his head. He did teach some basic conditioning but it was always very cryptic (for example he practiced very important and specific stretching exercises in front of his students but he did so as part of his morning prayer, so nobody picked that up). He talked about the physical principles behind the stretching, but using Daoist metaphysical imagery (that he learned from daito ryu but that originated in China) coated in terminology from Oomoto-kyo (= a small religious sect with incredibly complex mythology). For example, instead of saying "hold your head that way and let your body sink that way" he'd say "stand on the floating bridge of Heaven that brings together Izanagi and Izanami". It's indeed technical information, he'd repeat it often but how the hell were his students supposed to know what he meant? You'd need knowledge of both Daoist and Oomoto cosmology, as well as hands-on technical instruction.
And for trying to do the stuff myself, it is a freaking complex, boring and strict form of training with slow progress, even with guidance. There's no need to hide anything, as the secrets keep themselves and make you work damn hard for the slightest results. And Ueshiba further coated that with layers upon layers of mystical BS. Frankly, I don't think he was that interested in teaching in the first place and he just enjoyed the attention. When in a good mood, he'd show some stuff. If I understand the guy's personality (based on several biographies and interviews with subject-matter experts) he saw himself as some kind of half-god and therefore it didn't matter if people didn't get it, as long as he'd ascend to godhood, but hey if they managed to grasp some stuff good for them.
My theory why bjj is transmitted effectively was that it was advanced by the students and the competitors. More so than the masters.
So as soon as say someone started smashing fools with leg locks. Everyone ran out there and learnt leg locks.
Agreed. Competition provides an art's community with a common goal towards which they can work. Innovation is encouraged and successes feed the art. Ironically, aikidoka like to talk about spontaneity but aikido is one of the most stale martial arts around. And also, when one looks at iconic daito ryu masters like Takeda or Ueshiba, those guys gravitated towards other skilled martial artists and frequently exchanged with them. If they were alive today, there's a good chance they'd do MMA.
That's why I think Tomiki was onto something with his idea of competition. It provides a common ruleset to train your timing and distancing, deal with resistance, exchange, innovate, challenge yourself and bring together people from all lineages. The rules may not be perfect, but nothing prevents us from improving them over time.
But even here, there are big obstacles. The biggest and most authoritative aikido organisation (the Aikikai, run by the founder's descendants) is violently against competition. Part of this is due to the decades-old decision to position aikido as "the non competitive martial art" for marketing purposes. Part of this is for fear of losing influence (and the money of affiliates) to the bodies that would organise said competitions. And another problem is that the Aikikai is trying to tap into the funds of the Olympic Committee and would see the emergence of "sports aikido" organisations as a threat to its wallet.