Why indeed? What keeps aikido from being as viable in the same time span as judo, karate, BJJ, etc?
Viable in what way? How are you making the comparison? Each of those arts has it's own set of criteria in which to judge viability of effectiveness. BJJ in a judo paradigm typically does not do as well as well as vice versa. Same with aikido and BJJ, both seem like oil and water when you compare viability and success from each others methodologies.
It is sort of like we need to develop a framework that is generally agreed upon in which to test skills and viability. A set of conditions and test in which we could agree upon.
From there, we could view the various strengths and weaknesses of how things such as baselline skills are additive. Same wtih Judo, BJJ, Aikido techniques.