Maxis, well, remakes are often a large turn off to many consumers and monster games are big turn ons, and the sales as you just have shown are all over the place in comparison to the previous. Sure, SquareEnix is awful at marketing, but the localization hasn't caused any suffering on sales. Besides, I think the best way to view sale relationship is through percentages. In comparison to Japanese sales, Dragon Quest games have sold in America:
Dragon Quest VIII - 10% total sales
Dragon Quest Heroes Rocket Slime - 23% total sales
Dragon Quest Monsters Joker - 27% total sales
Dragon Quest Swords - 18% total sales
Dragon Quest IV DS - 11% total sales
(Before Merger) Dragon Warrior VII - 5% of sales
(Couldn't find gameboy color figures)
As a main series title, in comparison to Japan, Americans showed more interest towards DQIV than DQVIII. Spin offs Americans tend to show more interest in comparison to Japan (especially the Joker game). Swords and Rocket Slime didn't sell very well in either America or Japan. Crossing out the two lesser games, you are left with three games since the merger, with 10%, 27%, and 11%. That isn't really a downward spiral from the point of the merger... more of a parabola. While Joker didn't have the same localization style as most of the others... Joker wasn't a hardcore/traditional rpg either. It had similar elements, but it wasn't a DQVIII or DQIV.
I predict that Dragon Quest IX may actually sell more than Joker here in the states (possibly break 1 million), and I also predict it to have the same localization treatment as the Zenithian Trilogy. I also predict the "monster capture" aspect of DQV alone to cause it to sell tremendously. Normally when a Dragon Quest game doesn't sell as well in Japan, in won't sell as well in the states, Joker being the only exception (although it didn't do that bad in Japan, but for a DQ game it didn't do that hot).
Remakes are remakes, spin offs are spin offs and main series titles... are, well you get what I mean. The best way to compare figures would probably be to wait until we get some sales figures for Dragon Quests V, VI, and IX. Then we could compare VIII to IX and compare IV, V, and VI. Those numbers would be more accurate then what we have now. Currently, there are no real reliable numbers to compare and from what I can see, there are no real reasonable relationships between the figures. Curse Americans and their lack of interest in the remake of a NES rpg. Oh well, Dragon Quest IV sold better than Rocket Slime and Swords. Of course, it hasn't sold as many as a many as DWVII (yet) or DQVIII, but it didn't do bad either. It's just that wild card Joker that throws off the figures... I don't like wild cards (;_;)
No pun intended ;)
As for whoever is in charge of marketing Dragon Quest, this message is for you... you deserve serious punishment for your failures. I will find you-... err, I probably won't. I just hope that whoever you are, you will be fired for your failure at selling Dragon Quest!