The only way for them to regain trust is for top executives to resign, starting with the CEO. The breach of trust is so severe, you can't take anything they say at face value, and you can't trust them to not try this again.
This is the second time they've tried to change the terms retroactively, and this time they broke their promise to not try it again.
No, the execs need to be let go. This is what would happen to any non-exec that messed up so egregiously, they need to feel the pain they're used to inflicting.
Really the board needs to go as well; someone pointed out that the founder of Iron Source is now on the board of Unity. The CEO has been in place since 2014, but recent changes were probably motivated by recent board members joining. I'd say the board all need to be replaced for trust to be regained.
The board can only be fired by stockholders, right? Apparently, stockholders don't think this announcement is too big a deal, since the stock hasn't moved much (in a way that's distinguishable from noise).
They already did that during the Improbable debacle, and attempted to undo that safeguard here by sneaking in a retraction in a terms of service change months ago.
Unity has made it abundantly clear they haven't cared about the engine for a long time. They used to be working on a game to help identify areas where the engine could be improved and ended up firing the team who was working on that to "refocus priorities." There's also been the wave of lay offs for top talent and even more leaving voluntarily, of which Mike Acton of data-oriented fame was one of.
This is the second time they've tried to change the terms retroactively, and this time they broke their promise to not try it again.