Hire more people to do what? Be on-call?

Hiring more people is a great idea for making on-call less stressful, because it allows for much sparser on-call shifts (e.g., one week every 8 months vs. one week every 2 months). But I don't see how hiring more people gets rid of on-call completely. Someone will always have to be on-call.

You cannot just hire people who do on-call-only duties and nothing else, you have to be very closely familiar with the code and features being written in order to resolve issues effectively. Which usually means that you need to be someone who actually worked on those things, not just some glorified extra-fancy customer-support worker.

According to TFA, programmers at Amazon are required to be on call for a full week at a time. More programmers could mean sparser on call shifts, but it could also mean shorter on call shifts -- i.e. instead of going from one week every two months to one every eight months, go to two or three days every two months.

Being effectively at work 24 hours a day for seven days straight sure sounds like a recipe for burnout to me. As TFA notes, these people aren't doctors, the only thing that happens if a glitch takes two hours to fix rather than one is that Amazon loses some money. Amazon naturally would prefer not to lose that money, but its workers presumably would prefer to be able to sleep too.

If the concern is about performance during business hours in China, you should have employees in China. Or hire a night shift.

On-call will still rotate amongst the developers. They wrote the system, they know how it breaks and can fix it.