We've had to give up so much flexibility. Wayland certainly focuses on plugging this hole, but it means we've lost all these cool utilities like this one. There was just so much you could do with devilspie, xdotool, and others to make sure my operating system and window environment worked for me.
I still really miss X11's Zaphod mode, where you had two independent X sessions (:0.0 and :0.1) on two different monitors, with different window managers and different windowing rules.
I miss the days of being able to trust my computer and trust my software.
That's only true if you decide to trust it.
You can deal perfectly well with software you distrust, and not have it harm your system.
Thankfully, for a lot of software, there is no reason to ever give them network access in the first place.
Is that a reasonable argument against using X11? Sure, for some use cases. Is it a good argument for wayland/windows/OSX/whatever to do your tiling WM experimentation? Not really, those environments kinda suck for playing around with.
[1] Or "local-ish", your system or a trusted remote has to have been compromised already. Untrusted X11 protocol still exists but is deliberately disabled (and often blocked) everywhere. Even ssh won't forward it anymore unless you dig out the option and turn it on manually.
It's not like it's not a valid argument, just that it's sort of a nitpick. Security is hard, and defense in depth is a thing, but this particular attack surface is way, way back in the "depth" stack for a modern app deployment.
