It'll work, it just won't get quite as many conversions as it might otherwise. The same principle applies to closed-source software, after all: you have to provide the binary, so I'll just modify the binary to give myself an infinite trial. This is entirely doable, but enough people still pay to make it a viable business in many cases.

The number of people that would bother to do that is relatively small. If it's a good product, and reasonably priced, people would much rather just pay to have it. (See music streaming.)