Absolutely not true. I have 4 children some still teens.
The majority of teens do not do nothing and it isn't well known.
IME of teens, many maintain a sport if they have been encouraged and supported by their parents.
Mine still play multiple sports and love them, but they also enjoy and need down time.
The vast majority of their friends are similar. They play at least one sport.
The OP is rightly concerned about her husband never stopping.
Its not good for him and not good for the children.
Read her posts.
The children are over tired and cranky.
Not good.
It is wonderful for children to play sports.
However, over scheduled, worn out cranky children, will rebel at some point and ditch them as they get older.
They have to enjoy what they are doing to stay involved into the teen years, when they can't be forced.
I have a very active husband who is nearly 60 and whilst not as OTT as the
OP's husband, I have had to remind him upon occasion to take it easier.
Several very fit men in our circle have had heart surgery that they never expected to face.
Without exception, although fit, they were doing too much for too long, and have had a rude awakening.
Well done OP for having the hard conversation.
It had to be done, for the whole family.
His children need him healthy, not burnt out with long term compromised health, which would be a real tragedy.