Re: Dojo loyalty or personal gain?
Hmmm. This doesn't seem to me a simple question. As a student, your role is to learn from the instructor and work with other students to learn aikido. Yet, you are interested in leaving because your seniors left and your instructor is having difficulty keeping class energetic and fresh by him/herself.
Rocky made several great points about growing in aikido, and even though you are new to training, I have a feeling this is your first growing pain. If you leave, you may go to another dojo and continue to train much as you have been. Or, you can stay and help your instructor rebuild the dojo with new sempai and new energy.
Your obligation to your dojo is to help others learn aikido. The question is can you help others best at your current dojo, or by moving? I once asked my instructor why he taught class and invested time in our training. He replied, "Because someone invested time in me."
If you have a good relationship with your instructor, my advice is to buckle down and start learning how to help others in your dojo, including your instructor. If you do not have a good relationship, you may have chosen the wrong dojo to train at and should consider another location. Neither of these solutions breach etiquette with your instructor.