As I said, PC3 was given the go ahead before MadBox. They managed to fund PC2 without any issues - why do you think they can't fund PC3? If it was to be the launch title (don't remember reading this), that would be to sell MadBox rather than to sell PC3.
SMS are primarily a games developer. They were looking to get involved in console hardware. If they have changed their minds about the console (at least one source says they haven't) you seem to imply this means they would then stop developing games. I don't see the logic at all. If Microsoft, another software developer who decided to get into console hardware, changed their mind about making the X Box a few months into development, would they then say - guys - this X box idea hasn't worked out, lets cancell the next version of Windows and stop making Office.
I am not saying that they would stop making games obviously. Of course that wouldn't make sense and you are simply using a straw man argument here.
However, SMS might be a tad smaller than Microsoft and a seemingly troubled project of this scale can very well have an impact on other projects. It all depends on how much weight the MadBox sales had in the PC3 business case, really. It might very well have little impact (being mostly included in a best case scenario) but I don't believe it isn't part of the business case.
To be fair, when looking at the alleged reason for the investors pulling out and the amount of money involved, I doubt that the MadBox was taken into account with any significant likelihood. I would assume it was on a too shaky ground to be counted on for PC3 income. But then again, why announce a project like that to the world when it is not much more than an idea? Doesn't make you look particularly good.