Standards are important, and the meaning assigned to words are important, but if your very important email does go through, because Google thinks you're wrong, you add the bloody message-id. I really do agree, I don't care about the linguistic/legal/standard/technological reason as to why Google might be wrong, if I can't deliver email to Gsuite customers, I add the message-id.

You're precisely right that customer experience matter, but I wouldn't put it past some conservative European company to go: Well Google is wrong, so they should fix that. Google doesn't care, you can't make them care, you can't even contact them. Just make it work for your customer.