The working life of an MCU actor is a pretty blessed thing. You're handed a role in the world's biggest and most lucrative movie franchise and provded you're not playing a villain or obvious dead meat, you're pretty much guaranteed a retainer to come back several times. In other words, you're pretty well set for a few years of solid work and pay.
Obviously, that's not the only reason actors sign on to Kevin Feige's behemoth merry-go-round (with actors as big as Matt Damon and Peter Dinklage now willing to take cameos and tiny parts for the prestige as well), but it's certainly got to be a big bonus.
So imagine what it must feel like to have played a character in an MCU film only to find out that you've not been asked back for the sequel. That must sting - and not just in the wallet. So far, it's happened a few times over the last 19 movies and remarkably, in some cases, it was actually the choice of the actors themselves to step away. At least Marvel should know that it won't be too much of a problem to recast their biggest stars if they have to...
7. War Machine
Original: Terrence Howard
The Replacement: Don Cheadle
Back in 2008's Iron Man, Tony Stark's best friend Jim Rhodes was played by future Empire star Terrence Howard, who played the character straight and balanced Robert Downey Jr's volatility really quite well.
Sadly, when it came to renewing his contract for the sequel, Marvel apparently moved the goalposts - at least according to Howard himself - and the person he says he "helped become Iron Man... took the money that was supposed to go to me and pushed me out."
Howard claimed Marvel said they'd only pay him one-eighth the original contract offer, so he passed, leading to unfair claims he was greedy and Don Cheadle stepped in to make Rhodey more of a comic figure.