There were many Kabbalists after the Ari, but not a single one of them was able to understand the Ari’s method at the root. Only Baal HaSulam was able to do this because he received the same soul as the Ari. It’s not because other Kabbalists were lower than him, but because the Ari’s method can only be understood by a soul from the same part of the common soul, from the same “spiritual organ” of the body as the soul of the Ari. Other souls understand the same thing from the perspective of their organ, whether it is the spiritual heart, brain, liver or lungs.
Your soul may come from a very important spiritual organ; it may even be higher than the soul of the Ari. However, since he is different from you and has different qualities, you are unable to truly feel his method, to feel “at home” with it as if you are dissolving in water. You cannot feel it as your life unless you are built the same way he is.
Therefore, we do not judge Kabbalists according to the height of their attainment, but according to how much they can help us correct and ascend. This depends on the type and character of their souls and their ability to help us.
Therefore, there is no one we should study besides Baal HaSulam, and no one else we can follow in order to reveal the Upper World. After all, he was the only one who merited the same soul as the Ari.
Any person who feels an aspiration for the spiritual world and desires to develop, is part of one of the most important spiritual organs. The rest of the souls are there to fill up the spaces in the body between the important organs, and thus they carry out a passive role.