As a person who has done weight cuts for weigh-ins, I imagine weighing in at the gate leads to hungry, dehydrated people who get weighed and then immediately eat and re-hydrate. I'm not sure plan bathrooms are suited to the consequences of that.
The weighing could be done at the end of the flight.
It's possible some people will actually get healthier. However, it's also much easier to lose 10lbs from dehydration and pooping than from losing fat (from my experience).