Millimeter wave frequencies are just one aspect of "5G", the standard of 3GPP Release 15 (and newer) are improvements and build off the existing standards of LTE (releases 8-14). Its quite possible, and likely that "5G" will be deployed across all existing spectrum, including lower frequency. Much like LTE has been and continues to be deployed across existing spectrum that was 2G/3G.

The focus on millimeter wave is large part due to existing capacity problems in high density areas, or for high bandwidth applications.