I spoke with John Kassir (The Cryptkeeper of Tales from the Crypt) about this a few years back. Because their faces are not on screen, studios tend to undervalue voice actors. It is common for them to try to cut costs by bringing in a new voice. That happpened famously with The Simpsons, but few voice actors have the clout and platform as that cast. As I recall they tried to do that with Kassir by bringing in Charles Fleischer (Roger Rabbit - which brings a whole new vibe to the Crypt Keeper). Fleischer turned them down and gave Kassir the heads up. It is very much an honor thing with voice actors. As for Cullen and Welker, there is a slight difference here. There are a number of other actors who have voiced some version of Prime and Megatron over the years, so it is hard to peg them as actors who "own" the characters the way Tom Kenny does Spongebob. In fact, before Cullen was brought back for the films, Gary Chalk might have had more claim to Prime in terms of years. I am more bothered with them going out of union when there is probably a wealth of young and cheap talent, than with them replacing Cullen and Welker (both of whom I do have tremendous respect and affection for). Finances are a thing, though. I have no idea how Netflix measures the worth of its series since there are no ratings or advertising. It may be keeping the VA costs down were the only way to do the show. It isn't unusual for actors' escalating salaries to hurt the profitability of a series.