How so? Supply goes down while demand is the same --> Price goes up --> Profit margin for the farmer increases --> Profits of mustard > profits of rapeseed -->

How so? Supply goes down while demand is the same --> Price goes up --> Profit margin for the farmer increases --> Profits of mustard > profits of rapeseed --> Farmer produces mustard instead of rapeseed. This is exactly the consequence of supply and demand. The only noteworthy fact is that we got in this situation because it was not a completely free market but a regulated market due to subsidies for rapeseed, artificially increasing their profit margin.