Sabre posted - "After all, the three-fire team Marine squad was born of Pacific island battles where casualties were sometimes well past the 50% mark that would reduce even the mighty three-team USMC squad down to the strength of a single large fire-team."

The 13 man, 3 four man fire teams was developed in the Pacific as you stated. It has remained thru thick and thin the basic Marine squad configuration for more than 65 years. Longer than any other successful other infantry squad configeration since WWII.

In today's environment 50% casualties are not the norm, and that fact increases effectiveness of the 13 man squad.

There are a hundred reasons for the difference in squad size, and that will probably remain the case well into the future.

65 years is a strong indicator that the Marines might have found the answer and see no need to change their tactical
basic unit.