Those companies have a MUCH weaker stranglehold over global communications than the media companies of 30 years ago. I can publish a website and have it viewed by (almost) anyone in the world without going through any of those 4 companies.
If you had a video or book or article 30 years ago, you couldn't even distribute it yourself around the world without the partnership of a major media company. It is way easier now.
Did people consider this a danger to free speech in the 90s? I certainly don't remember it that way. No one was worried that you couldn't get a main stream publisher to publish your crackpot theories, or that no cable tv station would air your crazy rants. No, you were forced to self publish, and deal with the fact that your audience was a lot smaller than if you could get a major publisher/media company to publish your work.
The NyPost still has a website, and anyone can still go to see it. It is way more accessible than self-published work was 30 years ago.