My understanding is that this is possible: you should be able to take a C project, add a build.zig file and under the hood the system is calling clang to compile the C project. HOWEVER, you can now add a .zig source file, compile that in zig and link together with the output of the C compiler into an executable. If this is actually true, I can definitely see the attractiveness of the language.

Easy interop with legacy code is how kotlin took off, so maybe it will work out?

Good interop was a requirement for widespread adoption, but not the reason why programmers want to use it. There's also null safety, a much nicer syntax, custom DSLs, sealed classes, type inference, data classes, named and optional arguments, template strings, multi-line strings, computed properties, arbitrary-arity function types, delegation, custom operators, operator overloading, structural equality, destructuring, extension methods, inline functions and non-local control flow, reified types, ...

Some of these features have since been added to Java.