Cowell, 55, who helped to put the group together on 'The X Factor', said he fears for the five-piece boy band, reported Contactmusic.
"They can't carry on doing the amount of stuff they've been doing for long because they'll burn out - it's a high-class problem. But they don't complain about working hard," he said.
The 'Steal My Girl' hitmakers, who will embark on their third world tour next year, are "generous" to their fellow artists.
"They still come down to 'The X Factor', into song meetings and they will text me with ideas for the contestants. They're generous to other artists. You don't see that very often. Most artists couldn't give a damn about anyone but themselves!," he said.