Considering the actual complexity of rendering e.g. Urdu in decent, native-looking way you presumably do want some Turing-complete capabilities at least in some cases, cf "One handwritten Urdu newspaper, The Musalman, is still published daily in Chennai.[232] InPage, a widely used desktop publishing tool for Urdu, has over 20,000 ligatures in its Nastaʿliq computer fonts." (https://en.wikipedia.org/wiki/Urdu#Writing_system)
Edit—the OP uses this exact use case, Urdu typesetting, to justify WASM in Harfbuzz (video around 6:00); seems like Urdu has really become the posterchild for typographic complexity these days
Edit—the OP uses this exact use case, Urdu typesetting, to justify WASM in Harfbuzz (video around 6:00); seems like Urdu has really become the posterchild for typographic complexity these days