broadband clarification

I'm sure that none of you really think that a cable modem
is a modem

Correct, we know it is

since it doesn't do AD/DA conversion but we all understand
that it's simply a device used to connect a customer to a provider.

No, it's a device that converts your data into an analogue signal
(modem) suitable for carriage on broadband plant (cable)

After all "cable modem" is nothing more than a marketing term so that
customers have a rough understanding of what the device does (not that
they always understand that either).

No it's an actual device that performs the function correctly
denoted by it's name. Broadband is the marketing term.

The question then remains: What (in your opinion) constitutes broadband
according to the services that have been promised to consumers but not
yet delivered?

The rates obtained from current day broadband systems (e.g. cable
modems & *dsl) are normally what is being referred to by the contraction