You need to spend some time in some competent telecom books. Pay attention to the development of the switching network and dialing plans. The choice of 7 digit number was made because that's the ideal number for humans to remember. Early in the life of the network, the second digit of the 3 digit prefix disallowed the use of digits 0 and 1. That is how the area codes were discerned. The needs for additional numbers and the advent of electronic switching allowed the use of 0 or 1 in the second digit. That expanded the available phone numbers in an area code, but required dialing a country code first. The country code for the United States is conveniently "1".
Area code splits are a hell of a lot of work. I've worked a bunch of them during the years at PacBell. Doing an overlay is comparatively simple.
I remember when they did it in Los Angeles from 6 to 7 digits to increase the numbers.
You're old enough to remember that point in time. The original switching machines were designed for numbers in the range of 0000->9999. The digits above that were used to select the outgoing trunks to another switching office. The "magic" 3 digit area code with 1 or 0 as the second digit did the long distance mapping from area code to area code. The next 3 digits did the end office selection. The last 4 digits mapped to a customer line.
" The choice of 7 digit number was made because that's the ideal number for humans to remember."
Those people shouldn't have telephones.
I probably know 20 phone #s that I use, my SS#, bank account #s, etc. and never write any of them down.