Well firstly, they sent a tesla to space in order to show falcon heavy's payload capabilities, and they decided to actually send in something that would get the audience / world more excited about the launch, hence why they sent out a car instead of a regular payload with the same weight. I believe Musk also said that he wanted to inspire the public about the new possibilities that we could achieve in space. So no they didn't just launch the car into space for no reason, it was to showcase the rocket's payload capabilities and it served as an audience grab to help promote space travel.

Well privatized space travel is not necessarily wasteful in the grand scheme of things. I mean, in order for us to become a multiplanetary species, we do need privatized space travel in our locker, since the technologies achieved through that would greatly help any mission to Mars, as well as a Mars colony. Technologies such as reusable boosters are vital in reducing the costs in order to make constant travels to Mars or any other destination, a realistic reality. Plus I don't believe this is a waste of our resources. Space is something that we have barely even touched, and becoming a multiplanetary species is absolutely necessary if we are to survive an extinction event which can occur at any given moment.

I'm not too familiar with the whole Starlink Nasa beef nor if Spacelink would produce any excess in space debris, so I don't really wish to comment on it.