> It's always struck me as extremely bizarre that a company that regularly advertises that it's at the bleeding edge of software engineering practices (...) made such a colossal error as writing their own language
There are no other languages that could replace Rust for Mozilla due to their combination of performance and safety requirements. It is also the product of Mozilla's research arm, which is not directly focused on immediate productization and employs several people with PLT backgrounds full-time. Finally, Rust has always been an open-source projects and has developed a robust non-Mozilla community that contributes heavily to the compiler. None of these is true of Wasabi.
In any case, Spolsky still firmly believes that Netscape would have been better off today if they had continued work on the (completely unsalvagable) Netscape 4.x. Having extensively reviewed the history, Joel was dead wrong about this. There's no reason to assume his advice is more trustworthy than that of any other "thought leader," or that it is consistently followed within his own company.
Just like ... Rust.