Thankfully, bathrooms don't have to change overnight. They can be modified bit by bit as parts of them are proven to be a better alternative.
The law is already here, no need to modify it as we've got Document T.
The technology really isn't futuristic at all, an engineering student could make it as a side-project.
It's practically just a mmWave radar running on very simple logic gates, which is already being used in smart buildings.
You can tie it into a building's existing security or fire alarm panel, meaning you don't need to hire dedicated staff for it, just use what is already available.
You're right that resin doors are heavier and solid, but standard commercial hardware is already designed to deal with them.
The real issue, as with any improvements people bring up (and hell, there are so many brilliant ideas out there), is simply governments not giving a crap about doing the bare minimum to implement them. So many of these ideas could actually lead to less money being spent long-term and turn into an investment.