My main concern is that a commitment to never, ever break backwards compatibility is a commitment to obsolescence.
You started this sub-thread with:
"If I understand correctly..."
I'm not sure that you do...
Our scheme is, in your terms, basically:
<major>.micro
where <major> is a decimal.
You cannot reason about the whole and fraction portions of the decimal independently.
When <major> changes backward compatibility can be broken - with respect to both API and implementation.
It therefore makes sense to reserve room in the numbering scheme to be clear and honest about when backwards compatibility has been broken. The major number is the normal place to do that.
I'm not convinced there is enough risk here to compromise the present in order to accommodate some unknown scenario that may never even come to pass.