“New aircraft designs take even longer and cost more money to certify, which is why we fly on planes whose designs date back to the seventies.”
This statement is misleading. Designs are constantly being tweaked for performance, updated based on stakeholders (i.e. pilots, OEMs, airliners, etc.), and changed to incorporate new functionality. While some aspects may be similar to earlier versions, most new aircraft designs still go through the certification process as though they were completely new.