The problems that I'm seeing for staying forever in US are these:

1) Carer are expensive in USA. If you get old and become partly disable, who takes care of you? What do such old persons do in USA?

2) Medical costs. I was surprised that insurance meant sh*t when you have some big operations or health costs. You've to pay everything for health. And it's so expensive. Doctors are absolutely expensive. It sounds like it'd be cheaper to fly to Nepal and do medicals than to do medicals there. But can you come to Nepal once you have to leave your Nepalese citizenship? (Or does USA allows double citizenship?)

So, when I think like this, I feel it'd be more beneficial that someone work in USA for 30 years and afterwards, return to Nepal, stay here.

But it's not very practical either. After 30 years, your family, relatives, all will have moved on from you. You're really going to live the even more lonely life than you lived in USA. Because at least in 30 years in USA, you must have created 1 Nepali friend.
But most people who are here would've lived w/o you and I won't be surprised if nobody really values you anymore.

So, how are Nepalese youth planning with this situation?

US government will take care of you if you have a green card or a citizenship.
They will provide you minimum amount for food, lodging, medical etc, but you can easily survive with that on a small city.

Medical cost is slightly higher, but you don't have to go to hospital every year. When you have to go to hospital for a big surgery, there is still a limit amount 10,000 USD that you will pay out of pocket. Normally, if you are young, you can take cheap insurance with high deductible/limits. However, when you get older, you have to take better insurance, and pay more premium, but pay less per year if you have to go to hospital.