Someone can be both "likely to become a public charge" under the law (which is not yet in effect because three federal courts have ruled it unconstitutional) and also of great benefit to the country.
He's not an American, but Stephen Hawking, for example, was a "public charge" whose substantial medical bills were largely covered by the NHS[1] and the cost of his 24-hour nursing care was made possible through grants.
And while Hawking is an extreme example in both the cost of his care and his ability, a determination that someone is "likely to become a public charge" can apply to just about anyone who might find themselves in need of Medicare. An otherwise healthy 20-year-old with a family history of Parkinson's or Alzheimers. Or, to get even more dystopian, an otherwise healthy 20-year-old with a genetic marker for Parkinson's or Alzheimers. The cost of long term care for these diseases is astronomical but I'm certain there are many Parkinson's and Alzheimers patients that have made substantial contributions to society.
The law saying “likely to be a public charge” is already in effect and is decades old. The recent change that is currently blocked is only about the factors being considered when making that evaluation (specifically, usage of various types of government assistance).
That’s not quite true. You are probably thinking of the fact that when you sponsor someone for a green card you are required to promise that they will not use government assistance for x years, and if they do you will have to repay it.
I was always under the assumption that immigrants were suppose to benefit the native population. Which in this case they likely wouldn't?