The highest ilvl pieces are available day one. Getting full 16 slots would take some time and unless you are 2400 in pvp, and near full clearing mythic, it's not gonna happen in the first month.
Harder content is available instantly to casuals with the exception of mythic due to the guild requirement. If you got in a guild tho and proved to them u are good enough, u cld raid the first day of release with them.
As for your 2 points, there have been multiple points in this game where patches last too long and everyone gets full geared. You see a huge decline in players. Happened a lot in tbc and wrath.
Fully decked our gear has never been awarded to everyone. Never. If you are referring to wrath, the badges took months to acquire and you could only buy 4-5 pieces with it. Not anywhere near full gear. And the ilvl of it in wrath was that of the normal raid, not the 25 man heroic raid.
Casuals are the best geared now than they have ever been. Theres so much data to support that. So if you're gonna use current population statistics, its proving the opposite of your theory.