I have been told personally by a top executive at Maserati, someone I personally know and trust, that it has indeed been decided to upgrade the Granturismo and to keep it in production. The main reason was that the investment capital required to develop the Alfieri is not available yet, but as Maserati could not be seen as falling out of the GT market which must remain a fundamental part of its heritage, they decided to upgrade the Granturismo with a smaller investment and to keep it going.
I didn't hear about any limited production numbers, actually anything like that would be very counterproductive as they would never recover their investment by building only 400 cars, since they have to remain aggressively priced.
The two disappointments from my side are the failure to implement the excellent new generation ZF 8-speed gearbox and the equally wonderful 3.9 twin-turbo V8 of the Quattroporte GTS. A 600HP Granturismo would have been a dream come true for so many enthusiasts (and would have taken a lot of sales away from cars such as the BMW M6 or the various AMG Mercs), but apparently fitting the new V8 would have required an enormous structural redesign work for which there simply wasn't enough budget. So, the Granturismo will live on, with the venerable 4.7 normally aspirated V8 (perhaps with a minor power increase) and the outdated 6-speed gearbox.