I think Froome wins the Giro but digs pretty deep in the process, is vulnerable at the Tour and ends up with a battling third place. Which would still be a phenomenal result and arguably holding all 3 titles at the same time would be a bigger achievement than getting a 5th Tour win.
After thinking about it, I have a feeling that the sponsors are pushing him to do the Giro, and "attempt" the double. What I mean by that is for the last 3 years or so, Froome has been the favorite, and a lot of fans have been resigned to the fact that the same thing will happen like it does every year. We have seen it on this forum...how the TdF is predictable and boring.
While the Giro and Vuelta by themselves are not nearly on the level of the TdF from a worldwide sponsorship exposure perspective, if Froome shows up for the TdF with the previous 3 GT titles, that's all the media will talk about for 3 weeks. It will bring much more attention to the TdF and create doubt. And even if Froome falls flat and struggles to finish in the top 5, it will still be more successful from a sponsorship perspective than skipping the Giro and winning the TdF IMHO.
I also think that Froome does want to win the Giro just to complete his personal palmares. That he knows the Giro/TdF double isn't very realistic, but he's just spinning it off that it is his goal when really he's focused on going all in for the Giro, and just see what happens at the TdF.