I'm a big conservationist, but I'm so disillusioned by it these days. You're onto something here. We don't seem to have much data on their population size, but all the articles start from 1997 data when the population was at a peak. You go back a few years and the numbers are lower. This is a private organization and I don't see the criteria by which they mark a species endangered. They probably just make stuff up. Most creatures go through population cycles, and just quickly looking at the data going back to the early 90's it seems monarchs have huge boom and busts. A monarch lays up to 500 eggs in a week... so if you have a good year with lots of food and low predation and 100x increase it not hard to understand. It also not hard to anticipate huge declines the following season.