Usecases doc makes incorrect claims about the needs of new PKI designs

[nharper]

Once the proposals like MTC have been developed into workable designs, they will need to ship with their own negotiation mechanism.

MTC needs to negotiate 2 things:

1. That the client and server support MTC
2. Which MTC CAs the client trusts

Item 1 is most appropriately handled with the signature_algorithms_cert TLS extension. Item 2 is handled by a trust negotiation mechanism (Trust Expressions or Trust Anchor IDs). Deploying a new PKI design doesn't need a negotiation mechanism for that PKI design - it needs to signal support (signature_algorithms_cert) and indicate whether the client will trust a cert that the server has that uses that new PKI design (a trust negotiation mechanism). Absent a trust negotiation mechanism, each new PKI design needs to build and deploy its own trust negotiation mechanism.

The deployment or non-deployment of Trust Expressions or Trust Anchors does not have any bearing on practicality and effectiveness of these next generation proposals.

The deployment of a trust negotiation mechanism has a direct bearing on the practicality and effectiveness of new PKI designs, as it solves a necessary and shared problem.