I think there's something a lot of Sonic fans don't get when it comes to Forces and its development cycle.
SEGA itself has suffered a lot back in 2014-2015, leading to a huge budget loss and a giant restructure of the company. (thread)

A lot of people were laid off to keep SEGA away from bankruptcy. Some of them were members of Sonic Team. A lot of focus was shifted from console titles and CGI film production to PC (Sonic Lost World) and mobile games (Sonic Runners).
Sonic Team worked on both of these titles.

The point is, the development of Sonic Forces is still not well-known as well as we think it is. Various sources claim that the game took 4 years to develop, some say that two, some say that one. But even so, here's my final verdict on this entire situation: