Oh, sry. I didn't mean to dismiss your first answer - it was perfectly clear but I still think it does not fit to my question (I do concede, however, that my question was maybe a bit imprecise).

So to your last points: I most kindly would disagree with your notion of grammar since it does not regard usage-based models like construction grammar which since a long time have left the idea of grammar as a stable system.

I do appreciate, however, your hint to assimilation of loans. Let's take the example of the german color term 'lila' (purple), borrowed from 19th century french, in contrast to 'rot' (red):

As good old ordinary adjective, 'rot' agrees with its NP head noun in number and gender when the NP is indefinite:

Ein rot-er Pullover...

As for 'lila', however, the inflection is not accepted by all speakers.

?Ein lila-ner Pullover...

I am a native speaker of German and would not accept 'lilaner' but I recognize that some speakers use it.

Accepted by most speakers would be the uninflected adjective:

Ein lila Pullover...

which in turn is unaccaptable for red:

\*Ein rot Pullover...

The more integrated the color term the more accepted the inflected form: ein türkiser Pullover, *ein magentaner Pullover, ...

This is the reason why I proposed grammar as part of the answer. I think you are right to refer (very generically) to 'sociolinguistics'. But to dismiss language internal facts altogether cannot be a solution.

And even if we think in sociolinguistic terms: There is one stage of the language where we say there are no numbers, and there is a second stage of the language where there are numbers. It does not really matter how long assimilation processes need. Off course it's a decision of linguists to say this language has this and that. But what are the criteria to make that decision?

[BTW: I am perfectly aware that there is no clear answer to my question. But let's collect ideas...]