... They both have a place I feel and making them rifles might negate choice because a 5 man, accurate on the move unit in itself if great (see rifles who are designed to operate with no support at all and stg volks who are designed to make support obsolete)
Both riflemen and Volks have access to weapon upgrades the suggestion for 5 men grenadier is that are about riflemen level and are there to protect LMG grenadiers from being rushed.
They will simply not be able to operate on their own because they will lose to allied infantry that have upgraded with weapons.