actually under that scenario a whole different german would likely develop instead of hoch-deutsch, the dutch area would still have the same huge economic pull, so lots of influence language wise.
as pompejus already wrote regional culture and language was more important earlier. that is why i stated that in the light-green areas dutch was used as lingua franca (trade language), while normally they would speak their own low-german dialect.
dutch and all the german dialects were around at the same time, although the differences with dutch in some areas were a lot less, with the introduction of hoch-deutsch a cultural harmonisation started.
it is for a reason that the netherlands first name was 'the united provinces', the provinces were more important than the country.
i say the divergence started around 1200, until 1400 they were still officially part of the HRE, but during the Burgundian era the process started to forge the netherlands into a nation, a process that was completed during the reign of Charles V with the pragmatic santion of 1549.
https://en.wikipedia.org/wiki/Pragmatic_Sanction_of_1549