The maesters may well have poisoned the dragons. (Or perhaps they didn't.) If they did, their reason may well have been that they believed that this would prevent the continuance of magic. Neither proves that they were right, though. One might argue - I can't really be bothered to (and its a weak argument anyway) - that the maesters were able to poison all of the dragons because the magic that may have protected them was growing weaker.
As to the wildfire, it is true that its increased in power was noticed after the hatching of the dragons, but that doesn't prove causality. Was it used just before the dragons were hatched? Was its efffectiveness less than we saw later? We simply do not know.
(By the way, I've only read one of the novellas, The Hedge Knight, so I have no idea whether any other information is out there on this matter.)
As to the wildfire, it is true that its increased in power was noticed after the hatching of the dragons, but that doesn't prove causality. Was it used just before the dragons were hatched? Was its efffectiveness less than we saw later? We simply do not know.
(By the way, I've only read one of the novellas, The Hedge Knight, so I have no idea whether any other information is out there on this matter.)