This is called timing the market, if one can do it as perfectly as you describe it, they would be better off with just trading and not having to deal with all the mess associated with mining gears.
The coin trading markets are of course efficient, meaning it's impossible to time the market. But I don't think the mining equipment market is perfect. There are always plenty of suckers who buy video cards even when they know proof-of-stake is scheduled for July and has been going according to plan. I might regret purchasing the Antminer L3+'s in the end, even though they are at the cheapest price in the past year.