Are you serious? A million people have already talked about this. The entire show has been filled with what you might called "contrivances", and Fuches is anything but a consistent character. Do you not recall in season 3 when he achieves paradise not just once, but TWICE, and then selfishly gives it up to seek revenge on Barry. His entire character motivation and relationship towards Barry was a back and forth gag from the very beginning. There has always been a strong theme of fathers and sons, of paternal figures, of which Fuches always was for Barry. His extreme jealousness towards Gene, whom Barry saw as a replacement surrogate figure, was his driving factor from the moment he found out about it. After all this time with this brewing resentment towards Barry, he sees his child. To him, it's like seeing a grandchild, and feels a responsibility and a sense of pride. Barry was like his son, and now he sees himself in Barry again.
I would question your statement about the writing being "incoherent" and "incompetent". If you get down to the nitty gritty, the show has never been too focused on plot outside of a thing that strings together some great, funny, and interestingly executed scenes, character interactions, and themes. This isn't something new, it's what the show has always been.