If someone succeeded in building a third-party access to crypto that only worked for the US government, the immediate effect would be the abject failure of every product incorporating it, since all foreign governments would automatically prohibit internal use of all such products and anyone who needed to interact with those systems would have to not use those products as well.
In the little detail of the recent Florida school shooter, not only did law enforcement not react to his public posting of plans to attack the school, they didn't react when it was specifically reported to them. The FBI somehow forgot to investigate entirely when they received reports of his postings, and the existing supervision he had been under ignored the fact that their previous decision that he didn't need any intervention because he was in school and living with his mother was no longer valid because of him having been thrown out of school and his mother having died.
Actually, Apple and Microsoft are a poor analogy for the F-35 communications system failure, since the current and recent OS versions from them can communicate over a network without any difficulty at all (and have for a long time, actually), and they can also read each other's disks with easily available software. Since they both run on the same processors now, it also isn't excessively difficult to make the binaries for one run on the other, needing only a suitable OS call translation layer. (In other words, precisely WHY is anyone running a Mac these days? It's the same performance you could get under Windows about two years ago for about twice the price of a current Windows machine.)