Why are monarch butterfly populations booming?

If, like me, you've enjoyed seeing so many monarch butterflies around, particularly this year and last, then you might want to thank ... the weather in Texas.
The monarch migration, which has been described as one of the natural wonders of the world, is coming to a head here in southern Canada as, after several generations of butterflies have lived and died over the last few months, one particularly long-lived generation sets off on the epic 4,000 km trek down to the mountains of Central Mexico (with much lesser populations heading for the Yucatan Peninsula, Cuba and coastal California).
After a couple of decades of poor (and worsening) numbers, resulting in an estimated plunge of 80%, last year and this year have seen a large uptick in recorded estimates. There was a 144% increase between 2017 and 2018 alone, although bear in mind that this is 144% more than what was a very low (and unsustainable) baseline. This year's figures won't be available until the Mexican government/WWF count towards the end of the year, but they area at least anecdotally, expected to be good (which bodes well for our trip to these them in Michoacan, Mexico, next February). Having said that, the monarch still remains on the "special concern" list - not actually endangered, but in danger of becoming endangered.
So what has changed in the last couple of years?
A dinner party guest of ours recently insisted that it was all down to human agency, that people have been planting more milkweed (the monarch's favourite, but not exclusive, food, and the only plant on which they lay their eggs), and the banning of various pesticides like neonicotinoids (a known threat to various pollinators and other insects). This all sounded convincing enough when expressed in an authoritative tone, but it also seemed a bit too simplistic to me, too good to be true. As I suspected, the proposed ban on neonics in Canada has not yet been implemented, and a final review on the proposal is not expected until the end of 2019. The province of Ontario has introduced regulations reducing the acreage treated by neonics, but a complete ban would cone under federal jurisdiction. A ban on neonics in the USA is even further away from reality (an Obama-era ban on neonics around National Wildlife Refuges was reversed by one, Donald Trump). So, a (non-existent) ban on neonics is clearly not the reason for the current resurgence in the monarch population.
Well, the latest theory is that monarch numbers are most sensitive to the weather, and in particular the spring weather in Texas, which is usually the first egg-laying port of call on the monarch's multi-generational northward migration from the oyamel fir forests of central Mexico. Warm weather in early spring in Texas, creating prime conditions for milkweed growth, is strongly correlated with good summer numbers of monarchs in Canada, and vice versa. Last year's weather has been described as "Cinderella weather" by one scientist, and a "fluke of weather by another, hence the large surge in monarch numbers.
So, scientists are cautiously optimistic over the prognosis for the monarch butterfly, and it is good to know that they are able to rebound so impressively when the conditions are auspicious. But it would only take a couple of years of poor weather, particularly severe storms, to decimate their numbers all over again, which in an era of global warming seems more than likely. In fact, the twin threats of climate change and habitat loss remain the greatest challenges to the monarch's survival (among other effects, increasing carbon dioxide levels may be making milkweed too toxic for even monarch caterpillars to tolerate, and warmer temperatures are encouraging migrating monarchs to venture further and further north, leading to more dangerous migrations). And the other thing is, we still need to be planting and encouraging milkweed, because the bottom line is: no milkweed, no monarchs.