Yeah, I've seen a decent amount of that already happening. I have 2-3 of all the duals already, but am nervous about stockpiling tons because it feels more risky (imo) than many people credit. Duals were not expensive before SCG started the legacy opens, and if they replace the legacy opens with modern, it is very possible that duals could drop back down to like 40-100 instead of the current 80-250. I know I personally would abandon legacy except maybe keeping a pet deck (BUG pod in my case) if SCG dropped it. Lots of people say SCG won't ever drop legacy, but I'm not sure how realistic that is. Heck, they broke 20k viewers on their stream this weekend for the modern GP... there is obviously a lot of interest in modern.If fetches reach those prices, I am flipping every goddamn one of them into legacy staples like real dual lands. There are a fraction of the number of ABUR duals out there as any of the fetch lands, and they're never coming back. Hell, Tarns and Mistys already sell for more than about half of the duals do. That's just nuts.
It also feels like there are other things to trade for that will appreciate much faster (as % ROI) than duals even if duals don't drop. Most recently I picked up 30 spellskites at $9 to $11 and 20 birthing pods at $5 to 8$ and those are going to appreciate way faster than dual lands. Heck, spellskites are already up 50-75% and Card Kingdom is buylisting pods at $18.20. Obviously not going to get rich with that, but I feel like there are always 2-3 obvious cards like that which are going to notice significant appreciation and can help keep the hobby affordable if you buy/trade for a bunch while they are cheap.
I guess you were probably just making a point about how absurd the fetchland prices are, but soooo often I see people treating dual lands as the gold standard of guaranteed value in MTG and I don't think it is anywhere near as solid as many believe.