FWB is a made up term (by men), to make women think that they are "cool", because they are having sex that is empowering and "for feminists" etc, whereas in reality, it's men getting sex for free, without ever having to as much as take you on a date, buy you dinner, or do anything that a boyfriend would reasonable be expected to do for a girlfriend. Oh, and they can cheat on you as well, because, you know,.......it's just FWB, and they told you from the outset that it was only ever just a shag.
Who benefits? It's certainly not the women. Women are biologically programmed to fall in love with men that they are regularly having sex with. It is literally hard wired.
Your post proves my point really, because it's obvious from what you write, that a FWB success story for you, would be where the man suddenly has a lightening bolt moment and realises, that actually, he doesn't just want to shag you and several random other women, he actually loves you, and wants to marry you and make babies with you.
Any guy who tells you that you're just a FWB from the outset, is an opportunistic fuck boy, who doesn't want marriage and babies (yet), and he is literally spelling this out to you in neon 6 foot letters (at least he's honest), so my advice would be to walk away and start dating men who at least start out on the premise that you could be a girlfriend or wife in the future.