The first couple of million users at least won't be bothered by launch software not being particularly brilliant. Across the last few decades, only a handful of launch games have been excellent games. Strong launches are based on hype and the appetite for new hardware, not new software, and that's what helped to drive Wii U and 3DS despite both consoles having their fair share of issues at launch.
PS4 has the benefit of months of positive PR and news coverage, in addition to launching at the tail-end of a very, very long console cycle, after the first new console to market has failed to achieve much market traction, and with Microsoft having months of mixed and negative press globally. It's essentially been a perfect storm of public relations and press for Sony, with their rivals struggling to communicate their hardware and software effectively, while Sony have played their own game near perfectly. For the core consumers that buy at launch, all that press and bluster matters immensely. This isn't to underplay PS4; Sony seem to have corrected every major mistake they made launching PS3 and that deserves plaudits. But more impressive to me is how Sony have captured so much positive attention and kept it focused on PS4, which has in turn been helped by the constant negativity around Wii U and the stumbles Microsoft have made pre-launch.
Sony will have strong sales until the end of this year, though I'm not sure how things will play out after Christmas. I'm not expecting any kind of Wii U style drop off, but that 5 million target could be a tad too high. What's the software situation like post-Christmas? I know Destiny is coming across PS and Xbox, and Titanfall is coming to Xbox. Do Sony have high profile exclusives coming? I'm not sure this mountain of goodwill and hype can push PS4 to six strong months of sales, though less surprising things have happened.