Of course there will be people who continue to use the toilets that match their gender id, not their sex. Plenty of trans women are being quite vocal on that front.
Like with eg wearing a seat belt or speeding, the rules are clear and we are supposed to obey them, regardless of the fact that they can't be 100% monitored. It's up to individuals to decide how to behave.
So yes some trans people will continue to transgress. If they pass so we'll that nobody notices, they will get away with it in places where they are not known. Different at work say, or in a small community where people know. And if you can see under the doors, you can see if someone has their feet pointing towards or away from the basin.
But yes, there will be people who get away with it for whatever reason. This doesn't undermine the rules though. Most people hopefully will comply and it sends out a strong message.