I don't see DiDio saying that he "was surprised to find that Dick has a large and vocal fanbase." His words say that he didn't realize one basis of that fanbase: how people identify with Dick's growing-up process. Of course, that's the element he'd emphasize now as Dick changes his role.
In the years before Infinite Crisis, Dick Grayson's maturation had seemed to stall. Or, perhaps more accurately, fans weren't buying the journey his writers were taking him on, either in feedback or in sales trends. So DiDio didn't see what he represented.
But DiDio must have known that Dick Grayson was an iconic character, or else he wouldn't have pushed for his death at the end of Infinite Crisis. You kill a few minor heroes at the start of a crisis, one or two major ones at the end.
Ironically, the quality that saved Dick from last-minute death there was his connections to most other DC heroes. Yet in becoming Batman he left the Titans, and Tim left the Teen Titans, and it's not clear what's happening with the JLA. Furthermore, becoming Batman threatens the light-heartedness that's also part of Dick's appeal.