Even as a well known "windows hater", this is hyperbole. It's not insecure by design really. In fact in principle it's a lot better than anything Unix side due to the ACL and security model. It did however exist before anyone gave a crap about security, was implemented in a vastly insecure language and runtime and grew to a huge size and surface area and that is hard to fix retrospectively. I'll give Linux a stab her…
Better is subjective. It's maybe more capable. It's also easier to create confusing DACL structures that do not do what you think they do or leave surprising gaps in coverage.
> and grew to a huge size and surface area and that is hard to fix retrospectively.
We added network cards to everything at the same time networks finished moving from being isolated to being permanently globally interconnected. Machines that had a multi user capability (that didn't cost thousands) fared this transition better than those that did not.
> Anything which can read ~ is a problem because there's where all my important shit is...
It wouldn't matter if it's ~ or not. The software runs as you and so can access anything anywhere you have rights to. Which is why chroot, namespaces and pledge all exist, and windows really does not have equally secure equivalents to these technologies. To the extent it does, commercially available software does not seem to take any advantage of it.
Then again, running an agent that actively screen shots your desktop periodically and then saves those unencrypted to any part of the hard drive is a bad idea regardless of the imputed confidence you have in the security of the operating system itself.