Most of us don't have the funding, skills, time, priority to rewrite all of our old web forms apps even if we wanted to. Who is going to pay to rewrite the app that only earns us $10K a year?
Even if we did decide that Rust is great, do we immediately expect our C++ senior devs to produce the same quality code in a language they have only just learned? Are we going to recruit some confident youngsters who possibly know Rust but haven't been burned enough times to be pragmatic when implementing it in the wild. Do we want to rely on a new language that ticks lots of boxes but also lacks the understanding/support and battle-testedness of C++?