Honestly, with how Kojima left Konami in the middle of the production of mgs5, pretty sure Kojima just wanted to make something other than a mgs game. With mgs5, he made one of the best stealth simulators hands down. But, it felt the least like a 'mgs' game, especially a 'snake' game. I could definitely feel death stranding vibes from it though.
That's my guess why he ditched David in mgs5.
Pretty sure he was just sick of only making the same games over and over. Especially after he made 4 since it is obvious to me that he put everything he had into finishing the story of mgs.
He was itching to make death stranding for a long time imo.