It is hard do come up with a definiton of dialects that doesn't also apply to languages.
Intercomprehensibility is an elastic notion, which depends as much on what your own language is and how accostumed you are to dealing with other ways of speaking. We can probably agree that languages should be
less intercomprehensible than dialects, but there are so called 'dialects' which are harder to understand by speakers of the supposed standard language than officially recognized neighbour languages - especially if spoken by old persons with bad teeth. Linguists speak about linguistic continua, where A understands B, and B understands C, but A doesn't understand C. And if it weren't for migrations and wars and politics and mass media and other nasty calamities the world would probably be covered by such continua. Or in other words: If B didn't exist, then A and C would probably be seens as speakers of different languages. But what if both A and C get under the cultural wings of B? It is in that situation that their speech variants will tend to gravitate towards the speech patterns of B.
In the golden age of Low German (up to around 1500) it was obvious that it differed from the German speech variants further South. As Josquin mentions it has NEVER been a variant of High German (as defined by the second Germanic soundshift) - on the contrary it developed independently from Old Saxon, and Platt is far from being the only dialect which actually has had an independent existence along the supposed mother tongue - the idea that dialects have split of from the one and true mother language is in most cases plainly wrong. But then things started to happen which undermined Platt and most other German dialects: Luther wrote his Bible, the Hansa league lost influence and ... well, Low German speakers who needed to deal with people in other parts of Germany adapted their speech in order to be understood. However most ordinary farmers, peat diggers and fishers continued to speak their old dialect until they got radio and television, whereupon there was a phase were you could hear anything from hardcore oldfashioned Platt to something that sounded almost like HIgh German, just with an added "moin".
And it is in this situation that I take the hard stance that the soft soup in the middle simply isn't the same thing as good old hardcore Platt with all the bells and whistles of a true language or dialect. Sometimes it is nearer Platt, sometimes nearer High German, but as time goes by and the media do their thing the tendency is that people drop the characteristics of Platt and end up with something that really just is High German with a slight varnish.
The situation may be different in Switzerland, but then I would just wish that the Swiss had been less secretive about their local dialects during my visits there.
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