"Do you expect women to start getting undressed in their seats?"
Unless you are the Queen, being greeted by officials with thousands of cameras, etc, travelling in first class, (and with probably larger sized toilets), then the ordinary Joe Public would have quite a job to change in the toilets, which are very small.
The mixed sex toilets usually have 'wet' on the floor, and are very cramped, and unfair to the people waiting behind you. Just as it is your turn, the seat lights go on for descent.
So, in my view, it would be better to change and put on make-up, once off the flight, and in baggage reclaim, where there are usually toilets.
I agree that cleaning one's teeth is acceptable after a long flight.
The OP described someone spending 20 minutes in the toilet with a queue outside, and this omits those who see the queue and wait in their seats until the toilets are free.
So a bit unfair to other passengers.