I've dealt with many worse cases than this, where the systems I was integrating with were doing things that weren't even close to reasonable, but they had the market power so I sucked it up and dealt with it for the sake of my users. Maybe Google's wrong here, but how do you not just implement the solution anyway?
But they just did (make it work). The logical assumption is that most ppl did the same, just used another email provider. Why would viva care? (same as google, why would google care?).
This is the major issue that most of the discussion is missing. It doesn't matter how you want to interpret the word SHOULD, if you want to send to google workspace, you MUST include a message-id. It's not like this is some fly-by-night server with 12 clients.
If you absolutely and completely don't want to include the message-id, then you need to have a warning that your service can't be used by Google Workspace customers. This used to be common practice, blocking communication to servers that behaved badly, and I sort of wish we'd bring it back.
Chances are that the decision-makers in most companies don't care about the technicalities (i.e. which email you used for registration) - they want to get up and running.
The reason that Viva doesn't care, I assume, is the reason Google workspace doesn't care: they're both too big to care for 5% of their clients won't do the extra work. They know that their, usually much smaller clients, will "figure it out" by i.e. using another setup that works™. So why bother?