They are just middlemen. It is a good thing that they are going away.
It results in power prices for consumers.
And all 3p sellers are not middleman. Some are manufacturers and others are simply willing to take a smaller margin or are even selling used or refurbished products that people like me usually buy when given the opportunity to save a lot of money.
This doesn't make any sense though, as an argument.
If I am selling to 3rd parties, it is also true that I can set whatever price I want.
So, if I set the price to 5$, no 3rd party will be reselling my product below 5$. So I can still set the price...
On the plus side, 3p vendors contribute by getting a product from 0->1 or 0->100 units per day. This is what the industry calls "prospecting", and confirming that there is product-market fit for some product, and a lot of times marketing it in a way that causes it to sell. Supporting the customers, troubleshooting the various issues along the supply chain... this is something that a 3P seller usually contributes to (since a lot of times the manufacturer is focused on designing and making large quantities of product and not really focused on the rest of the supply chain).
This up-front work basically gets paid over time till it becomes rent-seeking behavior, at which point it gets disrupted by a direct relation between vendor and manufacturer. And basically the marketplace needs to hash out what's "fair" from prospecting to rent-seeking.
Ronald Coase wrote the paper that first got to the heart of this, one of the most important economics papers ever published: https://en.m.wikipedia.org/wiki/The_Nature_of_the_Firm