Probably it will be the non-portability of IMS applications.
If the application is in DB2 we can use ODBC/CLI(call level interface)
to migrate it to any plat form.Only thing is it should be compiled and link edited with those API's. I don't think IMS is not good in that regard.
Another point is the flexibility of the data types DB2 provides. We can store Large Objects(LOB), Graphics etc which i believe not possible in IMS.
More over in the new generation computing the concept of federated database is upcoming, to which DB2 can stick, but IMS can't.