...It would not be logical. That isn't how logic actually works.
Logic would factor in the changes in install base*, the changes or lack thereof in marketing, and other factors. You cannot just compare numbers from random periods in time, under different circumstances (especially when a publisher change occurs along with marketing) and somehow come to the conclusion that one variable is somehow "so-called". Piracy most certainly stymied the performance of DQVI and DQIX, marketing and larger/maturing install bases to a degree also would have worked to offset difference in epochs for the DS's software performance. This, however, does not change the reality of how bad software retail was becoming for the platform (and the PSP) entirely due to piracy. (I provided you some choice resources one page back.)
If anything, given the growth of the title and its strong marketing push, logic would dictate that had the DS not been suffering from crippling piracy in the late 2010s, DQIX would have performed better than it did because the ratio of sales

opulation:marketing balance is totally out of whack. Unlike it predecessors, DQIX was a new title and pushed strongly by Nintendo.
*The DS grew by ~20 million in the Americas from 2008 to 2011. The sales growth from DQIV to DQVI can be explained thusly even in the face of rampant piracy. The convolution of these things is impossible to separate. However, external details point to a market that was heavily impacted by piracy of software for the PSP/DS platforms. To then some how claim that piracy is "so-called" is to willfully choose to ignore reality.