A sole vendor who is not trustworthy is not a source of trust. Microsoft has done little to earn trust other than to make money from users.
It's always amusing how much people don't understand either of secure or trusted boot and start rambling about it.
OTOH, once the original Microsoft-signed SecureBoot keys for both Windows and Linux became compromised in recent years, triggering the need to blacklist those keys in everyone's firmware which requires an unprecedented worldwide need for a timely firmware update only if available from the original motherboard manufacturer, along with corresponding OS updates to match, neither of which has been fully accomplished yet, there was no-one to rely on other than Microsoft to mitigate the snafu.
More than just amusing, to "quote" Ballmer: "This is by design."
> "source of trust" is a technical term, and not a judgement about trustworthiness.
No, its both. And when there is a mismatch then you have a problem.