In fact, consonant shift can be viewed as a means of blocking umlaut alternations in nominal paradigm, e.g., the 5th declension in "-e" has front vocalic endings ("-e", "-es", "-ei", "-ēm", etc.) in all cases except pl.gen. which has the back vowel "-u" and pl.gen. happens to be the only case where consonant shift takes place for this declension (the 2nd declension in "-is" is not as immediately obvious because the modern pl.nom. ending "-i" is a front vowel which should not require consonant shift to block possible umlaut, however, it likely originates from an earlier back vocalic ending "*-ai" explaining the consonant shift.)