Why is insecurity 'inevitable'?
Subject says it all. It doesn't seem so inevitable to me.
Given that many of the light-weight realtime OSes you might find running instruments such as oscilloscopes often have little security [1], an obvious, simple and cheap way to secure the instrument would be to fit, say, a RaspberryPi model B inside a spare corner of the case and use it as a built-in network front end. For very little money this would provide a firewall and a reasonably capable login mechanism in addition to acting as a GUI for the 'scope. As a bonus it could also buffer and queue output sent networked printers and plotters or support one or two USB connections to local devices.
[1] I used Microware's OS/9 for several years. Its a capable and very reliable OS both for desktop and realtime uses, but security? not so much apart from a login and file permission bits which are there as much for keeping the idly curious out and protection against fat fingering: you can easily run it in single user mode if you want. In this I don't think its all that different from any other small realtime OS.