It's worth mentioning that all these measures can be fairly trivially defeated by the analog loophole[1]. I suppose it's harder to prove authenticity in that case, however.
It's not possible to do what Gmail wants to do without a SCIF.
https://en.wikipedia.org/wiki/Sensitive_Compartmented_Inform...
If not, consider that you might be making your own broadly wrong assumptions.
The thing that a lot of these measures protect against is not so much a targeted attack, it's stupid user tricks. It's not protection against Jane the Spy extracting as much information as she can, it's a measure against Danny the drunk who leaves his laptop at a bar or sitting in the back seat of the car where it's visible and stolen.
There are also likely a lot of places where it would be illegal to use something like this with auto expiring messages, though hopefully most such places won't be using Gmail.
This could be a useful feature when dealing with PHI, legal, HR, etc.
This seems like it should be true, but having worked with end users in the past I would not take this for granted
Disc: Googler