Virtual functions defined purely in ddl can be inlined under a given set of circumstances.

It should be driven/controlled by a hint/option - never inline, aways inline, or only inline after pushdown (similar to our handling of things like concat2 where it's best to pushdown the original function rather than the compensation).
the static analysis of whether the function can be inlined should be done only once - that it's just a return statement with an expression. The parameter expressions would need to be checked to see if anything is non-deterministic (or probably if it contains subcommands) and to disallow the inlining if the parameter is used multiple times in the expression (which could also be determined ahead of time). The rewrite could then handle the appropriate expression mapping.