At least with external entities you could deny the parser an internet connection and force it to only load external documents from a cache you prepopulated and vetted. Turing completeness is a bullshit idea in document formats.
With SVGs you can serve them from a different domain. IIUC the issue from TFA was that the SVGs were served from the primary domain; had they been on a different domain, they would have not been allowed to do as much.
calling Leonard Rosenthol ...
Postscript is pretty neat IMHO and it’s Turing complete. I really appreciated my raytraced page finally coming out of that poor HP laser after an hour or so.