Could be legit #4: The WWE is crowded with talent
One thing is certainly true; In recent years, WWE has made a concerted effort to sign as many huge wrestling stars as possible.
From Shinsuke Nakamura and AJ Styles to indie scene darlings Keith Lee and Ricochet, the WWE currently has one of the most stacked rosters in the history of pro wrestling--possibly THE most stacked roster since WCW had legends like Macho Man Randy Savage, Roddy Piper, and Hulk Hogan under contract.
Given that there have been so many new hires, Dean Ambrose might be wanting to head to a new promotion where he can be a bigger fish in a smaller pond. Add in the fact that many of these newer hires, like AJ Styles, have passed Dean by and entered the main event picture. This could be a reason why Dean's departure is for real.