Given how much of that $700B is likely to go towards developing such a system - likely somewhat less than all of it - and the general care towards usability in DoD-built systems? I think it might not be a wildly unreasonable position to take. Doubly so when you consider that DoD procurement and development practices are not widely renowned for being quick.
With those in mind, and that Signal already exists, it's worth considering if there's actually something for the DoD to gain by sinking millions of dollars and years of time into developing and maintaining something new that might not be an improvement. The answer might legitimately be "No, that's not worth it".
With those in mind, and that Signal already exists, it's worth considering if there's actually something for the DoD to gain by sinking millions of dollars and years of time into developing and maintaining something new that might not be an improvement. The answer might legitimately be "No, that's not worth it".