On the subject of salaries, and I have no idea if this is factually accurate, but it makes sense:
Marvel Film Star Salaries & What Comes Next
Hemsworth signed on originally for a relatively small salary, which makes sense. I don't think he was a big star before Thor, so why wouldn't he seize the opportunity? He needed the MCU more than the MCU needed him.
He subsequently renegotiated for the latest Avengers movies, which included appearing in future films.
I also don't think Marvel/Disney were too concerned about Downey Jr. & Evans salaries getting bigger over time. After 10 years and numerous movies, they were probably happy to pass the torch. I believe that if they wanted to continue their characters, Disney would have no problem paying up.
This isn't like Pirates, where Depp was getting more and more money but the movies were earning less and less. Marvel keeps getting bigger and bigger. Even at $75 million, there is sufficient return on investment for Downey Jr.'s salary.