I am curious if you have ever actually read the US Constitution because the federal government was clearly given the power to provide for the general welfare. While one could argue that one can promote the general welfare without providing for it, it's rather hard to provide for it without promoting it.
Yes, I read it. Have you read the parts I included that put limits on federal powers? While Congress has the power to provide for the general welfare that does not mean it can do whatever it wants under that provision that it does not have the delegated (or implied) power to do.
And, the courts have never interpreted that provision to give Congress unlimited powers--it is basically tied to the power to tax and spend and allows Congress to spend for items not specifically delegated but not control those functions. That primarily applies to powers reserved to the states such as education. Congress can give money to the states for educational purposes but it cannot carry out that function.
President Trump believed it was in the general welfare to prohibit travel from Muslim countries. In your interpretation he would have the unlimited power to make that decision. But, the courts ruled it was unconstitutional.