Actually, yes, kind of like how the PSP lost third party support (look at the amount of games released over the life of the system vs. other handheld consoles, excluding the abortion that is the Vita -- several developers have specifically stated that they did not localize or release games in the US for the PSP solely due to piracy on the system). Another example is the Dreamcast. The system was hacked less than a year after its release, and self-booting games that didn't require a bootdisc were out shortly after. Once it became as simple as downloading an ISO and burning it to a disc, the number of developers releasing for the hardware dropped substantially. I'm not claiming that it was the only reason the Dreamcast folded, but it was pointed to by several of the developers as a reason why they did not release games for the console.
The only time that piracy hasn't had an almost immediate and direct impact is when it requires the physical purchase of a chip and installation by someone slightly more knowledgeable than the average. For example, the PS1, PS2, PS3, Xbox, Xbox360 and Gamecube. All of those consoles had hacks, but they all required purchase of a separate device (excepting early model PS3s with firmware never updated past 3.55, those can be hacked with software only, newer firmware requires a downgrader device be purchased) -- be it a swap disc, mod chip, action replay, drive programmer, etc. Due to this, the prevalence of hacked consoles is notably smaller than the prevalence of hacked Wiis or PSPs. As such, the impact of piracy on these systems is notably lower due to the lack of a freely available tool to modify the console -- and this results in the lack of third party drop-off experienced by the other consoles I mentioned.
When piracy can be done at home easily with no additional purchase through software alone, it does have a notable impact on whether third party developers choose to develop for the system. When third party developers have openly come out and said this themselves, it's not really an arguable point that it doesn't at least contribute. It may not be the straw that breaks the camel's back, but it is definitely a contributor. Considering how poorly the WiiU is doing so far, I don't think it needs the straw.