Anyone ever have a FWB situation that ended nicely?

Yes of course... often.... but for men it tends to be about needing to let go with good grace when she finds something more suited to her needs..
I always say fwb tends to be
Short term...rarely works long term because:
either the woman falls in love and thinks its more.. cool if the guy is up for it but not cool if not.. always ends badly that one
Or the guy falls.. ok if the woman is ok with that
OR the woman sees him as literally short term *practice* and comfort while she seeks something better and more suited to her needs
AND/OR she is seeing others at the same time.. well there's no relationship so she doesnt feel tied.. and he gets left last on the list or not getting it that often... that can end nicely IF he accepts with good grace.. .
Unfortunately what ruins most is the woman falling.. we are just prone to falling for someone we have regular sex with..we cant seem to help that.. one way to reduce that is to make sure as a woman you have several fwb and dont just stick to one, oh and continue to look elsewhere too :P