I think money is a factor, either as a wage from Hyundai or using it as a bargaining chip with Toyota. But also, if Hyundai has a new car next season, there's a chance they will be telling stories about how many hours and €€€ they've put into development, and look at our facilities and engineering team etc etc and that could also be an element.
Personally, I don't want it to happen, because having the top three drivers in different teams is good for competition. Reminds me slightly of when Hirvonen went to Citroen, it neutered the fight.