Was there a substantial increase in retail price for FF13 compared to previous FF main games that justifies the decline? Don´t know if you remember but FF13 is also a game that had seen very deep price slashes incredibly fast due to performing way under expectations. DQ9 also sold so much because they chose the right systems. I also doubt comparisons with DQ8 and especially DQ7 will end that much better. Anyway it is just a matter of how steep the decline will be compared to DQ9.
The more interesting thing is, what happens after DQ11, since a speculated PS3 version of DQ11 in 2016 will be responsible for the lion share of sales, it basically is the backbone of why DQ11 is even beeing considered for PS4 at this point in my opinion. Unless a DQ11 successor launches quite early this very backbone won´t be viable anymore, that´s the point were DQ really risks even further decline, since I doubt PS4 will pick up the slack adequately.