The problem with the Japanese "securing their defensive line" shows up in the early Pacific War US counter operations like Mankin Atoll. On that island the US launched their first offensive expedition against a Japanese held island. Although intended as just a raid with no intention of staying put, the US demonstrated to the Japanese several things that were more than a minor shock to their belief that their defenses as currently set up were more than adequite to stop a US attack.

First, the US forces got ashore virtually intact. This alone was a rude shock, particularly after the drubbing the US gave the Japanese at Wake.

Next, the US forces took the island and pretty much wiped out the Japanese defenders in short order. Again a bit of a shock. Since the Japanese also did not fully understand the size or composition of the attacking force (due to their loss of the island), they were at a disadvantage intelligence-wise.

Lastly, the Japanese had banked on the US either not trying to assault islands they held or if they did being repulsed with heavy losses. At Mankin both beliefs were thoroughly shattered. It was now apparent to the Japanese that the US would try and assault islands they held and that the US had a good chance of succeeding.

Guadalcanal drove this lesson home with massive force. Once again a Japanese unit was overwhelmed as at Mankin in no time at all and the island lost. This time the US stayed put and then beat off every Japanese attempt to retake the island. Obviously, something needed to be done and done immediately.

The result of the Mankin raid and Guadalcanal was that Japan embarked on a crash program to upgrade their island defenses. Most SNLF forces on Pacific islands received more men, a great deal more heavy weapons, and more coastal defense artillery. These units were instructed to start construction on heavy permanent and field fortications as well as beach and island obstacles.

The difference shows up at Tarawa where the US was for the first time taking an island that met the new standard. Even then, the US won a somewhat phyrric victory. But, they won. The Japanese began to push harder to defend their remaining outposts.
But, even then some things were beyond their comprehension. The inner ring of islands like the Bonins and Saipan were still left largely undefended. These were truly Japanese territory and it was simply unthinkable that the US could ever get that far.

So, from the point of view of a heavy loss of carriers from the outset of the Pacific War, I really doubt that much would change. The US would have implemented liner conversions (outlined elsewhere here), built escort carriers, pushed far harder to get the Essex class in commission, and would have meantime still pursued the Pacific War with the means at hand.
Mankin could still have occured. Guadalcanal could still happen. Midway was defendable against a carrier air strike and certainly against a Japanese landing. Elsewhere, not having carriers does not preclude use of land based air power in its stead.
I really doubt this scenario changes much of anything early in the war.