Also arguing that any one dialect of a language is the original source is idiotic.
Maybe with English, but with languages that have an Academy/Preservation Board regulated with that dialect [usually the Capital area, though I understand Florence is the place with Italy] in mind that make all the dictionaries and grammar books...less so.
But just because there's some academy "regulating" the language doesn't mean that that's the original source or some sort of ur-dialect, which is what that phrase implies. Oftentimes, dialects can be more conservative than the standard [1], as is the case with English. Many Appalachian dialects retain a three-way distinction in demonstratives (yonder) which is quite an archaic feature, for example, even though, as far as I know, speakers of GA or RP rarely use it.
And in the case of Spanish, although it is true that seseo is a change, it is also true that it is much more common than distinción in the Spanish speaking world (including parts of Spain)--particularly in the US context. And it is accepted by various of the "academies" of the Spanish-speaking world. To me at least, distinción should be used mostly by those with a primary interest in Spain.
[1] This should not be overstated though. All languages change.
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