I agree that 3 days testing seems absurdly low.
I also agree that there's no way the simulator can replace testing getting to know the nuances of the car.
BUT
It's the same rules for everyone. It put a cap on the arms race where rich teams would spend thousands of hours testing that the smaller teams couldn't hope to match. The budget has turned the sport into a money making venture, (we might agree or disagree if we're happy that the teams are making money/in the sport for profit instead of love of racing) so that F1 will continue into the foreseeable future.
Also, part of the sport is testing the skill of the drivers. If you can get to grips with your car fast, that's good, and if you can't, that's bad. It provides a variable, it makes things interesting for fans, teams, drivers and sponsors.
The only area I'd be concerned is on safety grounds. If teams are trying radical designs that work on the simulator, but fail IRL, they need time to figure that out. McLaren has a new suspension design. Looks great, provides an edge, is it going to last the rigors of three races? What happens if it rains? I don't know, McLaren doesn't know. What happens to the new aero in a cross wind like we get in Mexico? That's a rough thing to find out on lap 30 of a close race.