They generally aren't real proposals in the first place (see hyperloop). They're usually vaporware used to frustrate a real transit proposal, analogous to e.g. deflecting proposals to move to a memory safe language by saying that you're going to use a safe subset of C++ that's going to be defined and standardised real soon now.
Excep c++ is standard, we hake alot of it and plenty of people who know it. You have instead made the counter my arguement. Sometimes new is better and you need to move on. However the safe c++ subset (if it arrives and works as promised) might be better because we have all that c++ and rust doesn't interoperate with it well. (again rust would be better if we had it in 1970 but we didn't.)
