Honestly, you probably are right, haha. What most likely happened is that Roger and Cindy had to dip out since they couldn't afford to keep working for the pay they were receiving. And I mean, Roger clearly did want to keep voicing Sonic. And also, Mike Pollock said that if he was leaving or not remained to be seen, so I doubt SEGA themselves were booting the VAs off. SEGA probably raised their pay once they saw the demand for Roger and Cindy's performances online.
I've seen a very simmilar situation happen with Miraculous Ladybug. Numerous VAs started dropping out of the show due to low rates (And the fact that different VAs were being paid different rates by the company, and they weren't allowed to discuss their pay) and the company raised their pay once they started getting backlash.
All speculation on my part, though, haha.
Roger and Cindy aren't union as far as I'm aware, at least.
(Speaking of VA Union, little fun fact: Kid Icarus: Uprising is one of the few Nintendo games to have union VO. Due to this, in Smash, all Kid Icarus characters aren't voiced by their original VAs with the exception of Pit and Dark Pit, who's VA goes uncredited in the game.)
Edit: Correction on that! Viridi's VA also reprises her role (albeit uncredited) but only for the Smash 4 Palutena's Guidance conversations. The new ones in Ultimate are a new VA.