Not really that surprising. Whedon has said he was tired of directing team superhero films and Russo's have been rumored to direct it ever since. I do hope Whedon stays as a writer thought or at least as a creative consultant.
Unfortunately, Whedon's said he has no desire to write Infinity War. It's too much of a logistical nightmare given all the characters and I imagine its a pretty thankless job, no matter how hard you work trying to incorporate everyone in an interesting/cool way, there's always someone who'll whine about X having less cool things to do/funny things to say than Y. He'll probably have some role in shaping ideas, but I think his time with Marvel is pretty much done.