Nobody is saying it's impossible to sell your product without going through one of the big platforms, just that it's expensive and difficult to market your product without doing so.
Also, I don't think it's ever as simple as established companies being "old and slow" so startups can "disrupt" them. Some companies are old and slow in certain dimensions, but at the top of the game in other dimensions.
Further, established companies have inertia, which can work for or against them.
But I have to point out we do hear things on this site like, 'the auto industry is old and slow", and I think it's false. Some of the companies are, and some are quite progressive. Industries are rarely homogenous.
On a social platform or online marketplace there is a natural tendency to cluster, customers do not want to maintain multiple social profiles or bookmark 10 "amazons".
I find this opinion (which is common on HN) hard to reconcile with the idea that established companies are "old and slow" and susceptible to "disruption" (also common on HN).
Like, no one can take out Apple's AppStore stronghold, but Tesla is going to bankrupt a century-old trillion dollar auto industry with a global supply chain infrastructure? Which one is it?