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!
It is really sad to see the retirement of cricketers from South Africa and West Indies. There are many cricketers who retire from their country's cricket just to play in foreign leagues. For this reason, many criticize these players, but if we consider their issues from the perspective of those players, then we will feel that what these players are doing is not wrong. The West Indies Cricket Board is not a developed enough cricket board, they cannot pay the players properly, many times such issues have come to the fore that West Indies players are coming to play the World Cup tournament on their own funds. Maybe no cricketer would want to ignore such a big offer of money and play for the national team for free, that is why they play cricket for the national team for a while and then when they get good recognition in foreign leagues, they retire from the national team and focus more on earning money.