Celtic's rate of differentiation

[A Rosta]

Steve Long to "JoatSimeon":
> You have no problem with proto-Latin arising among the other Italic
> languages before 500BC, wiping out those other languages and giving rise
> to modern Romance languages 2500 years later.

> But you do have a problem with proto-Celtic arising among related languages
> before 800BC, wiping out those related languages and giving rise to the
> Celtic languages 1000 years later.

By what process would proto-Celtic have wiped out those related languages?
By late migrations triggered by something other than the spread of
agriculture? If Celtic could spread without agriculture as its vehicle,
would that make Renfrew's model less compelling (--because it might imply
that PIE too could spread without agriculture as its vehicle)?

[Excuse my butting in, but you two seem to be exasperatedly talking across
 each other.]

 --And Rosta.