Doesn't matter if NASA is just one space org onboard with ISS, they contribute a huge amount to experimental science under the NASA brand which trickles down to Earth. The most immediate stuff going on won't be realised on Earth for some years yet just like the case has always been. The trickle down effect isn't always immediate but ISS has always been a hub for research to take place that cannot be done on earth. It is not money wasted by any stretch of the imagination and to say that the money could be used elsewhere is quite strange because the people funding these projects only have space research in mind hence why they greenlight the funding in the first place.
Elon Musk is privately funding stuff because he is a private individual and can do what he wants without the trivial aspects of an organisation having to greenlight everything, sign off for budgets and get collective bodies around the world onboard with projects. That's how private businesses work. NASA and the efforts on the ISS will result in direct downstream benefits to Earth as a whole whereas Elon's business will benefit his business interests only. Thai is the difference between privatised and not.
The long term plan for the ISS is to be converted into a public museum of sorts so that civilian visitors can visit in the future. A new collaborative space station is in the works to replace it. In between all that, various companies around the world will be expanding ISS and ISS will also remain as an jump-point hub to connect to other planned outposts in low earth orbit in the future.
Again, money not wasted...
You can find out everything you have asked with a quick google, I had read about the above in the past but a 5 minute google confirmed that what's said above is still relevant.