If you don't want kids then don't have kids. It really is that simple (as others have said). It's not your duty to society to have offspring. But do keep an open mind. I'm always surprised at how much my values changed from early 20s to early 30s.
I do have kids, and wouldn't trade them for anything, but the free time thing is real. I'm in no way jealous of people that don't have kids though. For me I still do everything that is really important, but I found I didn't have time for a lot of the non-important stuff that I enjoyed a little bit.
The cost thing is real too, but it doesn't have to be as extreme as most people make it. For us the only significant cost is child care and/or loss of income from 1 parent taking time off to be at home. Honestly, having kids can save money too because your values might change and you might save your discretionary income instead of spending it. It's not that you're making sacrifices, it's more like stuff you would have wanted (maybe a new car, expensive vacation, etc...) just isn't of value to you anymore.
Your other reasons are silly though (at least to me). The US economy is the strongest economy in the world. Who says you have to live in the US anyway. You can always move to a country with a strong economy if that's important to you. Who cares about social security. Would you prefer not to have lived if social security runs out before you turn 65. Who cares if the US becomes a 3rd world Spanish-speaking country. I've been to plenty of 3rd world Spanish-speaking countries and they are all great. I've even considered moving to one (briefly). Just because I choose to remain here doesn't mean I wouldn't be happy there. Those all sound, to me, like you're living in fear. Be positive!
I get the overpopulation thing, but that's not a good reason. If you don't want to increase the world's population than adopt. But honestly, your kids (no matter how many you have) aren't going to affect the world's population in any significant way (at least when they are born). Global population will only be affected by global forces. So unless you're a global icon and your decision to have or not have children is going to affect the feelings of many others, it's not going to have a difference. The world needs more good people who make a positive impact on society. Maybe your 1 kid will do something significant to reduce the overpopulation problem.