I believe that SLA is a manifestation of the General Adaptation Syndrome, along with any other type of learning, especially skills. This syndrome includes three stages which are predictable responses to stress in the human body. They include shock, recovery, and adaptation. The learner is introduced to a new stress in the form of the unknown language and eventually adapts after recovering from initial shock. This adaptation is not a vague abstract thought, but an actual physical adaptation to the learner's neurons and brain.
The brain itself will adapt given time and that the stress is not too large. A stress that is too large will simply induce exhaustion and overwhelm. A stress that is too small (i.e. not stressful), will not induce any adaptation and can actually allow for the learner to become unadapted as time goes on if a successful stress is not induced. This idea of unadapting is very similar to Ausubel's cognitive pruning, but again would be a very physical transformation where the neuronal connections simply become weaker over time as they are not called on to develop and strengthen. The entire theory actually fits rather closely with both Krashen's "i + 1" theory and Vygotsky's "zone of proximal development", however, this theory is rooted in biology and was development by endocrinologist Dr. Hans Selye.
One idea that does seem to be missing from Krashen's theories is that adaptations are specific and that, while transfer does occur among similar tasks, it is best to induce stress using an activity or task that is as close to what you hope to do as possible. For instance, taking a reading course for TOEFL preparation is more specific than taking a general reading course if that is your goal. Hopefully at least. The take away point with this last comment is that grammar does have a place in a theory based on adaptation, while it does not necessarily have a strong role in Krashen's. This is because some situations in life do call for highly accurate grammar use, such as academic environments and professional writings. For these situations, understanding that learning adaptations are specific means that time should be devoted them at some point. However, if a learner is not interested or does not have need for a highly accurate grammar system, there is no reason to study or focus on it. It will not necessarily help them with communication or further their goals in any way.