Yes, and if you vote no, I'll cut you...
Full disclosure: I do not currently own full playsets of all five original Onslaught fetchlands. I do not own any Bloodstained Mires or Flooded Strands, and I think I might not have any more Windswept Heaths either—I forget. I've mainly used my Polluted Deltas and the Zendikar fetchlands lately anyway. I could have completed playsets of all ten fetchlands months ago, but I chose to focus on getting Revised dual lands instead. I'll soon have full playsets of all of those, and they're generally more expensive than the fetchlands. So I'm someone who probably has/had hundreds of dollars worth of fetchlands and was going to spend hundreds more to complete full playsets of them. Despite all of this, I think I can reasonably assert that I'm not motivated by my own personal finances. I have a track record of vehemently opposing the Reserved list and the reprint policy in general. Even if I were to spend a lot of money on old card and then they were reprinted so much that the market value of the originals dropped, I wouldn't care. It's not about that.
Anyway, to elaborate on market values, the originals might not go down at all in this case, and here's why...
The original Onslaught fetchlands are part of a class of old card that I'll call "inflated." This group includes the Revised dual lands, Wasteland, Force of Will, Lion's Eye Diamond, Grim Tutor, and some other cards, but the fetchlands and dual lands are the biggest culprits. Many, many cards have high secondary market values, but inflated cards aren't like most of those.
With valuable old cards, especially ones printed before 1995, high price tags are associated with value to collectors. For example, the card Lich from Unlimited Edition is worth around $25 despite being a white-bordered reprint. It's a card that has essentially no tournament presence and that is arguably inferior to Nefarious Lich from Odyssey, which isn't even worth $0.50 on the secondary market. So why is a card that no one uses worth more money than many tournament staples are right now? It's because that printing of Lich was the last one ever, and this iconic rare has perceived value as a collector's item. It's part of the game's history. Very old, very rare cards are worth money because they're old and rare, not because they're good cards to actually use in decks. This principle is also why my Alpha Farmstead is worth more money than Jace, the Mind Sculptor (yes, really). For similar reasons, foil versions of cards can be worth a lot.
Back when the Revised dual lands had left Extended and were only tournament-legal in Type 1 and Type 1.5, they were worth less money than a lot of newer rares were. I remember buying some duals for $10 a piece. That's because Revised Edition had a huge print run compared to previous core sets, and collectors were only interested in the older, rarer versions of those cards. Two things changed that. Firstly, Legacy was established and became a successful tournament format, so there was a competitive environment in which those cards had utility. Secondly, the player base grew tremendously, and the previously more saturated market for dual lands introduced too much demand for the older prices. An Underground Sea from Beta is worth thousands of dollars, and no amount of hypothetical reprinting of the card (it's on the Reserved List anyway, but whatever) would change that. It's worth that money because of collectors, not because of tournament players. But the Revised reprint was never viewed as a collector's item, except maybe by budget-minded collectors who wanted full sets of old cards but couldn't dream of going for the really expensive versions. The price increase from $12 or so up to the $300 that Underground Sea is now worth has nothing to do with collecting and everything to do with tournament play. The manabases for the best decks in Legacy demand dual lands, and because Legacy grew so much, the demand inflated the price of the card. Force of Will wasn't even a rare. It was an uncommon. But it's so valued for its tournament presence in Eternal formats that it single-handedly makes Alliances booster packs worth more money than other booster packs from that era. It's a card that was printed as an uncommon and that is now worth over $90 because the print runs back then were so small and the card is so good in tournament decks that it's a staple wherever it's legal.
Anyway, a Beta Underground Sea would not drop in value if the card were reprinted in a new set. But a Revised version of the same card? People who want to spend large sums of money on the older, rarer versions aren't interested in the Revised version. It's only worth hundreds of dollars because it's a tournament staple and it's scarce. I see no reason why the secondary market value wouldn't drop if a new set (with a much bigger print run) reprinted the card.
But what about fetchlands? Well, they're a special case. The Onslaught fetchlands were still circulating as tournament staples in Constructed formats back when Legacy was established as a format, and they stuck around in Legacy and Vintage as Eternal format staples. When Legacy became more popular and the demand increased, the values of these lands increased as well. So they're kind of like the other inflated cards. But then WotC invented Modern, a format that didn't have access to the Onslaught fetchlands, but did have access to the Zendikar fetchlands. Due to a combination of factors, most of which can be attributed to WotC, Modern has become much more popular than Legacy. Even though Zendikar had a much higher print run than Onslaught, the fetchlands from Zendikar are nearly equally inflated, as there remains such a massive demand for them in Modern tournament decks. But when Khans of Tarkir becomes tournament-legal, the Onslaught fetchlands will be legal in Modern. The original versions of these cards may have some perceived collector's value, being older and rarer, and many Modern players, as well as Eternal players, might create enough demand for the original versions that they actually go up in value, rather than down. Or perhaps the gulf between the size of the print runs of Onslaught and Khans of Tarkir will suffice to make the demand for the inflated Onslaught fetchlands drop. I can't say for sure, but it seems clear that the popularity of Modern, where these lands will now be legal, is a new variable that makes this an unsual case compared to most previous reprints.
