That's just the "smaller" half the story.

> isn't this exactly the point of having a Mozilla Corporation

This is the critical part. If I am using Thunderbird (or the hypothetical mail service discussed above) in my company today and have an issue or request (bug, configuration error, need assistance with a deployment, want additional features in the product, want additional integration with other products) can I contact someone directly and know that this will be treated according to my specific expectations?

In other words can I call someone at Mozilla Corporation now and ask for dedicated tech support, for a bug fix in a specified time frame, for a consultant to listen to my needs and come up with a solution? Because if my business relies on that product or service and I get no guarantees then I won't be happy to pay you just so I can queue up on Github and raise issues, and I'll go with %prominent software vendor% who gives me that.

RedHat doesn't sell you "Linux", they sell everything that comes with it, and create an entire ecosystem to go with that (partners that offer certification, training, consultancy and support, dev support, etc.).