Oh, the turing-completeness is new, it wasn't in the past. Using Lua was to improve template performance.

Actually, it's been around for years - it was accidental. ParserFunctions is a good example of a DSL accidentally achieving Turing-completeness, at which point people start doing awful, awful things in it because they can. So they went to Lua because a proper programming language is superior in every way to an accidental one.

You actually have to be really careful to design a language that's both useful and not Turing-complete. Regular expressions are one example of such a language, but they are not strong enough for templating.

Yeah. It wasn't actually intended to be Turing-complete, but then some excessively creative goddamn lunatic came up with a way to beat MediaWiki syntax with ParserFunctions into yielding a truly hideously inefficient "if" statement. Soooo they gave up and codified that so the CPUs wouldn't melt.

I am afraid that the way those mark-ups have been used is the way they have encouraged.