Plenty of games used to be written like this, where you basically had a C (or C++) OpenGL ES platform independent code base, and then a bunch of small platform-specific parts for the different OSes.

They did usually have tiny pieces of code in platform-native language though.

I even wrote a game like that back in 2012 (originally for Samsung's bada OS, later ported to many things) https://github.com/peterholak/rail-climber (it says 2016 on github, but the code is much older).

One great thing about it is, when you have a desktop target, you can compile/test/debug things much faster (including things like level files), and only do a mobile build every once in a while to check that it still all works as it should. Almost like a full featured game engine.