Linux

The additional threat models Wayland guards against have never, to my knowledge, been seen in the wild (and may never be). So any improvement in security is hypothetical, and there are some tradeoffs in areas like accessibility. It's fine to make your own decision of which stack to use on the basis of hypothetical security improvements, but saying that X is insecure because it doesn't protect against something that never happens is dubious.

At the time when it was designed that was secure. Because the computer was probably in a locked room. And potentially as large as a room itself.