Some here has problem with reality.
Facts are:
1. 1.3 is used only for games
2. majority of games use only exec, trackdisk, audio
3. h may modify and sell 3.1 as they want
4. 1.3 is included in 3.1

Many overvalue Commodore contribution to amiga os.
exec 3.1 is as a whole in assembler as in 1.3
trackdisk 3.1 is as a whole in assembler as in 1.3
audio 3.1 is 80% in assembler as in 1.3

changes in trackdisk, audio from 1.3 to 3.1 are only bug fixes.
litte better is in exec, but exec 3.1 is still only a set of patches to exec 1.3