Part of the story here is that OpenGL ES isn't a new version of OpenGL, it's a different API for different platforms. It's nice if you can take old code and make it run on a mobile device easily but overall that's probably a small consideration compared to having an clean API that performs well.
If we want to focus on immediate mode specifically, it arguably shouldn't be used even in regular old OpenGL. It's slower and results in overly verbose code. Most people seem to use immediate mode because that's what most of the tutorial examples use.
Maybe one way to tackle the switch from immediate mode -> vertex arrays (and maybe quads -> triangles) is just to make some macros that take old code blocks and generate new ones.
Are you sure it's fair to call it a different API? OpenGL ES is officially described as "well-defined subsets of desktop OpenGL" [1], which would suggest it has a great deal in common with desktop OpenGL.
If we want to focus on immediate mode specifically, it arguably shouldn't be used even in regular old OpenGL. It's slower and results in overly verbose code. Most people seem to use immediate mode because that's what most of the tutorial examples use.
Maybe one way to tackle the switch from immediate mode -> vertex arrays (and maybe quads -> triangles) is just to make some macros that take old code blocks and generate new ones.