Many of Ueshiba M.'s students started Aikido with prior training in fighting arts and had the core mechanics for good fighting technique on starting Aikido. In this light one could just take them further along towards the "Aiki" aspects of the training. For those without that knowledge the (Aiki) Jujutsu type "fight science" foundation needed to be set. Even today this happens in my dojo, students who come knowing how to fight (especially from a Jujutsu/Judo background) just need to learn how to use Aiki strategy and tactics, balance control, timing, ma ai, kuzushi etc. more efficiently or differently than they did. Those who do not know this start at the ground floor.
It is primarily low-level Aikido that sucks in terms of effectiveness, as would be expected if Ueshiba was taking the strategy that I suggest. High level Aikido is quite effective and powerful, but takes a long time to learn. Also, if people take off on their own (as evidenced by the plethora of different Aikido schools) before they fully grasp the higher level, they do not have the basic mechanical advantages to fall back on, and so you end up with empty dancing about because the techniques do not work unless you can use them at the higher level, and no one knows how so there is basically nothing there.
This may be correct to a point but I don't think that the issue so much is teachers who leave before learning the effective stuff, since this (the jutsu aspect) should be addressed at the beginner levels imho before one gets into the pure timing, ma ai, kuzushi and internal power expressions exhibited at the higher level. Imho low level Aikido technique should resemble Jujutsu in free practice but it should be no less effective in how it operates. High level Aikido would be the clean, perfect timing waza that we often see in demos but done where the attack is real and the response is spontaneous. I think a lot of people use the whole "it takes years to learn" approach to explain away their own lack of understanding or lack of desire to do what is necessary to understand the sound physical application of Aiki principles. For those who have put in the effort the thing is just fascinating in its application and only gets better. I think the Judoka may have realised this also.
Just my thoughts.