It's not a warning not to build a compiler, it's more of a story about how feature bloat might cause one to inadvertantly build a compiler without realizing that's what they were doing. The last line reveals it by telling us that each of those "features" was actually a standard compiler component:

> Done at last, you say to yourself, without having to build a compiler.

> A parser, an intermediate representation, transformation passes, and a code generator. Dear Sir, you have built a compiler.