Nicholas Pooran, Heinrich Klaasen, Marcus Stoinis (ODI) are few among from the list of retired players in this year and their primary reason is to focus on T20 leagues because that is where the money comes not from playing for their national team so work load management is merely an excuse if someone say it so because they are doing it for years to get into this position and I believe work load management is mostly for the medium fast bowlers and everyone else can play all there formats with no issues at all.
That's easy for you to say. Try putting yourself in their shoes first. Any cricketer irrespective of what role they play in a team goes through a lot of stress and puts in a lot of effort and time to keep rising through the ranks.
Any smart person would prioritize work that brings in the most money. This is why those players that you mentioned chose those T20 leagues and retired from some formats in order to dedicate their full attention towards those leagues. Quality over Quantity!