Much like vim/neovim config, people want to do things like register a callback that fires to determine the window title and things that are much more advanced than that, like layout management.
When I was using sway, I was using its IPC or worse (https://github.com/johnae/persway) to do advanced things, and Lua scripting in the first class config would have been a much better UX.
Already has variable substitution and math, it seems to falls short of Turing completeness only because I don't see any form of recursion. But recursion is something it could accidentally gain if someone decided to add templating to the language.
Besides, looks the config files can call out to external executables, which absolutely are Turing complete. And moving to lua might actually simplify configs by allowing more things to be done inside the config system, rather than calling out.
I never found that Sway too annoying to just use the shell commands to grab/modify the current state of Sway (https://github.com/tombert/rs-swanbar) and just use a "real" programming language to actually do any kind of programming logic. To each their own!