People who say Michael didn't have an arc...
...are wrong, and bafflingly so.
Michael had an internal weakness, (mindless greed and ambition). He created an external desire (defeat of) which combated him against an antagonist (Hyman Roth) who knew how to exploit the weakness. This eventually destroyed Michael's humanity, his family and his criminal empire.
That's an arc. It may not be as viscerally interesting as Mike becoming the Don an a lot more subtle, but I found it a lot more harrowing and intense to behold.