What mission critical legacy hardware/firmware doesn't support TLS and is sending email to gmail?
Anyone running that mission critical hardware is free to keep using insecure email, no one is going to stop that. If they really need to send email from that old hardware to gmail, they can set up their own relay that accepts non-tls connections but relays using TLS.
Nearly every printer/copier with a scan-to-email feature in a small-to-medium size business that has moved their all their servers and email into the cloud.
Anyone running that mission critical hardware is free to keep using insecure email, no one is going to stop that. If they really need to send email from that old hardware to gmail, they can set up their own relay that accepts non-tls connections but relays using TLS.