Case in point? Who's insignia was adopted after the 5 year mission for all of starfleet? Kirks delta.
You mean George Kirk of the USS Kelvin in the 2230's?
The Starfleet Delta symbol was first seen (in-universe) on the Friendship-1 probe launched by the UESPA in the late 21st century (VOY: "Friendship One")
I like to go with the explanation given in the old Star Fleet Technical Manual
that the symbols designated different branches of Starfleet rather than different starships. It seems to fit all the retcons and goofs best of all.