"All devices are routeable" is a good idea, because it means when we want devices to be routeable there's a simple and obvious way that should be done.

Where we've ended up with NAT though is worse though: we still need a lot of devices to routeable, and so we enable all sorts of workarounds and non-obvious ways to make it happen, giving both an unreasonable illusion of security and making network configuration sufficiently unintuitive we're increasing the risk of failures.

Something like UPnP for example - shouldn't exist (and in fact I have it turned off these days because UDP hole-punching works just fine, but all of this is insane).