I'd say that's not a reasonable assumption, no. It's also not reasonable to assume the other direction. No numbers, you see. What's the statistical variance between people who would buy if no illegitimate source was available versus people who wouldn't?
Until we can actually put a number on that (By a third party, not involved with either the pro or anti piracy groups), we're pretty much pulling everything out our ass. Well, that and about a dozen other things related to the subject.
Is there even anyone reputable and uninvolved attempting to track numbers on all this shit, anyway? Sale rate before/after illegit sources are available, sales vs. unique downloads of illegit. copies, disparity between physical and digital distribution of illegit. copies, difference in rates between first and third world countries, probably the dozen other variables that need to be tracked to get an actual picture of what's going on? There's got to be a few academic studies tracking this shtick by this point.
Because both sides are prone to yanking vague numbers from "sources" (which they either won't reveal, don't exist, are being bankrolled by someone heavily invested in the results, and/or using shitpoor methodology) from their nether regions.
Basically, reasonable is nice, but reasonable is trumped straight to hell by statistics when you're dealing with this sort of thing. Let's get some reason trumping in this discussion, people!