Everyone always starts crying "contracts are running out", but MARVEL and the actors always find a way to keep them around. RDJ was only supposed to be in three Iron Man movies and three Avengers movies, but he's also been in Civil War, and now Spider-Man Homecoming. These actors enjoy working on these projects and will most likely always be happy to do more films.
That said, I agree that the MCU will be very different by the time 2020 roles around. The MCU will be 12 years old at that point, and if it's going to continue, then there will need to be some drastically different things. If Avengers 4 is the end and it wraps up everything, I would be happy with that too. By that point we'd have a bucket load of films, shorts, and TV shows in the MCU, which is DAYS worth of visual content, so if it were to end, I wouldn't be that sad.
Inevitably MARVEL will probably start the MCU Ver 2, but hopefully they let the MCU RIP in peace a bit before they reboot or whatever.