My U15 son has played club soccer since U7. Dabbled in a few other sports when he was younger, but otherwise was full time competitive soccer with DA and MLS Next. He was a starter on his MLS Next team and just quit. Would have liked him to finish out the season but given the circumstances with the 3rd new coach this season there was no point in staying. He was likely on a trajectory where he could have played college soccer on a scholarship, but we were never planning that as an outcome. The DOC of the club and an old coach who is now at another club are trying hard to convince him to not quit soccer as they claim he has a bright future. He's not persuaded at all. He was never passionate about soccer and played because he was good. He handles pressure just fine, but doesn't like it. He felt a lot of pressure from his coaches. It stopped being fun.
I think in some ways the term "burnout" is overused. As you mention, I think other opportunities and interests play a roll in that as well, particularly as kids go to high school. He will be going next year and plans to play football, basketball and golf, when he is not skating, surfing, snowboarding or hanging with friends. It takes a different sort of kid to stick with soccer. I was super bummed when he quit, but just I few weeks later I can say I'm enjoying having my weekends back and I have a new buddy for the slopes.
One thing I will say about club coaches. Most are very knowledgeable about soccer, but very few know how to teach, communicate and motivate kids.