If Meizu wants native drivers for the SoC, Mediatek will almost certainly not refuse. They're very big, and soon likely to be huge. They're also an incredibly valued customer as they're shirking Qualcomm entirely, and indeed it's their stated policy. Besides, if Ubuntu looks like it will take off, then having native drivers for their SoCs will be an asset for them in the fight to win business from OEMs intending to release Ubuntu phones / ROMs. But it is probably unlikely at this stage.
they widely distributed their code, competitors would appropriate it and use it in their own products within months.
Now that they're big enough, and are now competing with the big dogs, they can afford not to worry about knock-offs and code-stealing running them out of business. Hence why their policy has changed lately. Their policy was borne out of necessity, not because they want to lock everything down.
Personally I'm grateful that there is an emerging competitor for the virtual monopoly of Qualcomm, especially as they're not US based. Samsung as of yet aren't competing head on.