The arguments have been spelt out. If the logic is false, it can be challenged, but to dismiss it because 'we don't know' is to fob off all intelligent discussion. It's no different to discussing what may or may happen regards a hardware design, only instead of evaluating knowledge of hardware components, we're evaluating human behaviour. You don't need to be a mind reader to know a hungry guy is going to be thinking about eating food when some is waved under his nose. That's obvious. IMO it's only slightly less obvious that an artist (game developer) is going to prefer working on his own stuff than someone else's, especially when he's been working on his own stuff for a year.
If people accept that notion, then the argument that BT aren't particularly happy about this change is fairly sound. If people disagree with that notion, there ought to be a logical reason why a game developer who's had freedom to create their own stuff for a year would prefer to drop that and work on someone else's IP. I've even presented one above. But presently the discussion is mostly "BT probably aren't too happy about this" vs. "we shouldn't even be talking about this".
There's really nothing I can say to argue with this. There is nothing intelligent or useful about that line of discussion. You're talking about individual human beings. If you believe you can effectively determine another persons happiness without having met them, then good on you. You'll be a billionaire. Best guess I could make is that some people will be very upset, some will be happy, some will fall somewhere in the middle, and I couldn't put percentages on any of the three groups. Speculating at any more than that is going to be me talking out of my ass.