I think NIS's worries are more focused on how bad the Japanese console market is doing than anything else... Certainly, it does sound like Disgaea 3 & 4 both achieved over 150,000 sales internationally. Indeed, the fans of Disgaea in the west will likely pick up Disgaea 5 and probably already own a PS4, but there is a difference between 70,000 sales, which is what Disgaea 4 achieved in Japan, and 7,000 sales, which is what the typical PS4 game in Japan achieves. Compounding the issue could be that NIS cannot afford to make a game on more than one platform and that PS3 and Vita are pretty much dead in the West.
Assuming this is the case, NIS had to make a tough decision in either making Disgaea 5 for the Vita and pretty much losing out on sales outside of Japan, or making Disgaea 5 for the PS4 and possibly losing out on sales inside of Japan.
To summarize, consoles are in a bad state in Japan and handhelds are in a bad state outside of Japan, so a company that relies on sales in both Japan and outside Japan but can only support a single platform is going to be in a rough spot. By choosing PS4, NIS is actually focusing on the West and are hoping the Japanese Disgaea fans will buy a PS4 for Disgaea 5.
Assuming this is the case, NIS had to make a tough decision in either making Disgaea 5 for the Vita and pretty much losing out on sales outside of Japan, or making Disgaea 5 for the PS4 and possibly losing out on sales inside of Japan.
To summarize, consoles are in a bad state in Japan and handhelds are in a bad state outside of Japan, so a company that relies on sales in both Japan and outside Japan but can only support a single platform is going to be in a rough spot. By choosing PS4, NIS is actually focusing on the West and are hoping the Japanese Disgaea fans will buy a PS4 for Disgaea 5.