More aggressive in what way? The Japanese used their carriers primarily in hit and run raids against Allied strong points, sometimes in conjunction with invasion operations. But the Japanese had a number of problems to overcome before they could attempt any more operations than they actually did. The first and probably most important is the severe shortage of logistical shipping. The Japanese historically employed the equivalent of about 11 divisions in the first six months of the war in the Pacific, and this was pretty much the limit that their combined military and and merchant marine logistical shipping could support. If they wanted to seize more territory and hold it, they would have had to come up with more logistical capacity (which was impossible) or eliminate operations elsewhere.

Secondly, Kido Butai, itself was limited in what it could do. Kido Butai could not remain at sea for more than about 15 consecutive days (including traveling between their base and an operational area) and historically never exceeded this limit. That was not enough to project Japanese power very far into the eastern Pacific; The Japanese carriers could hit and run, but they couldn't stay and provide sea control. Additionally, the Japanese carriers began to feel the strain as the war progressed, crews and aircrew, as well as ships couldn't be run indefinitely without rest, refit and training. Most importantly, the carrier airgroups were becoming seriously diminished both in aircraft and trained, experienced pilots; at the time of Midway, IJN airgroups were significantly below pre-war levels and the Japanese aircraft industry wasn't turning out enough aircraft to replace even the modest losses suffered up to that point in time. The Nagumo Report makes interesting reading in this regard.

The USN wasn't going to "pull back to the West Coast" no matter what happened; Hawaii was identified as being part of "the strategic triangle", crucial to the defense of North America. As I have pointed out before, the defense of Hawaii had priority over even the "Europe First" policy. Had every single carrier the USN possessed been somehow lost, Hawaii would still have been reinforced, particularly with aircraft, to a level that would have precluded any Japanese attack. This would have been done by flying large planes in from the San Francisco area, and shipping crated fighters and short-range attack aircraft by freighter. The convoys would be fought through, if necessary, but the IJN did not have the capability to effect a sea blockade of Hawaii.

The sea lanes to Australia would have been more problematical, but as a casual perusal of a globe will reveal, it would be impossible for the IJN to blockade Australia, as well; the Japanese just didn't have enough ships and planes to blockade an entire continent, although they could make it difficult to continue the buildup of forces there. One has to understand that the Japanese were seriously over-extended even before Coral Sea and Midway and the more territory they captured, the weaker they became. They couldn't keep all their garrisons supplied with even the basics, let alone what it would take to hurl back an Allied offensive. The proof of this is that very soon after the Allied invasion of Guadalcanal, the Japanese gave the island a new name; "Starvation Island".

As I have argued, it is extremely unlikely that the USN would ever lose more than two carriers at one time (historically, it never lost more than one carrier in any single battle), but assuming it did lose two carriers at Pearl Harbor, it still has five fleet carriers which it can operate in the Pacific. Historically, the USN was often down to only one or two fleet carriers, yet it held the IJN to a checkmate throughout the first year of the war.