If Aikido were beneficial to everyone then everyone would do it and no one would ever quit. Since not everyone does Aikido and most people quit I'm willing to bet the Aikido experience wasn't all that beneficial to them or even beneficial at all.
Pretty simple actually.
The argument that if something is 'beneficial to everyone then everyone would do it' is untrue. A healthy diet and regular exercise is clearly beneficial to everyone, but does everyone have these?
People do not quit their diet plans because they do not find them beneficial, most quit because they lack the discipline or will power to stick to them.
Although there may be many reasons why people quit Aikido after the first few months, I would think lack of self-discipline would be far more common than beginners finding no benefit from Aikido.