I didn't say that experience alone is what makes a good pilot but it is still essential. You don't want two pilots with no experience flying an airliner no matter how well they did in flight school.
I also agree with you that college grades aren't the best metric but it's a start and it allows airlines to select potential achievers. And yes, I've been a slacker in my life at times too but you learn to pick yourself up and get serious if you want to achieve.