Um. No. No, it is not time to fire either of them.
This year is a huge disappointment at the major league level, no question, but it is important to note that players like Larnach, Ober, Jeffers, Kirilloff, and Alcala have been acquired by Falvey and Levine and/or come up through the development system that Falvey and Levine implemented, and they all look awfully competent. That's more good young players this year than the previous FO produced during their last ten years.
They took Byron Buxton and turned him from a strikeout machine full of wasted potential and turned him into a guy who is outpacing Mike Freaking Trout this year.
They haven't hit on all of their free agents and trades, especially this year, but they did go get Nelson Cruz and ride him to almost 60 home runs and two playoff berths in his first two years. They did turn Eduardo Escobar into a solid pitching prospect during a dead-end year, and then develop said prospect into what sure looks like an ace in the making. They did turn a middling shortstop prospect into Jake Odorizzi and then get the best year of Odorizzi's career out of him, and then they got the middling shortstop back for nothing and have turned him into what looks like a pretty solid young player.
They've taken a minor league system that was completely bereft of talent and turned it totally on its head. They now have a minor league system with at least a rotation and a half worth of guys who might legitimately be #1s or strong #2 MLB starters sooner than later. There are a handful of quality middle infield prospects and more outfielders than we know what to do with.
No, Falvey and Levine aren't perfect. This year sucks noodles for the most part at the major league level. They're playing the long game, though, and the team is clearly much better off with them than without.