I should stress that that is also your misconception. For example, you can implement generics without an AST if you don't care about performance; codegen can either output a single binary mandatorily requiring a sort of vtable (therefore every parametric code will be indirected), or output a partial binary with holes where holes will be patched later (much faster but disallows optimization across hole boundaries).

You should have framed your question as follows: those features can be implemented without an AST, but an AST is a standard and reasonable way to do them and not using an AST would require a strong rationale. So what's that rationale? (And amedvednikov, this is my question for you.)