This is like the argument that Amazon has a monopoly, but it's actually even weaker. Doordash is just picking stuff up from local vendors that buyers could visit themselves, whereas Amazon is shipping people things from all over the world.
Amazon has been pretty successful in arguing that Walmart, Nordstrom, and everything in between are their competitors, and I expect that DD and IC will have no trouble doing the same (especially under this administration).
I wouldn't be the one to ask about the details, but I think it would be foolish to surmise that two companies taking up 3/4ths of a sector shouldn't go through some anti-trust scrutiny.
While they are an effective duopoly, it would be good to also consider them as monopolistic competition versus a true market split out -- there is very little traditional moat, and almost only network effect due to restaurants accepting their services and menu.
But economies of scope, such as they exist, can be good for consumer welfare (unpopular as that is to discuss in current public forums).
Two chunks of overhead (IT, web app, mobile app, finance, HR, etc.) instead of one can be bad for consumers too.
But yeah, flexing pricing increases to gobble up consumer welfare is what the FTC is supposed to rally against, and, while services expand their markets aren't that different.
EDIT: Wow, people really hate this comment! I'm not being normative here. Economies of scope are the thing that any acquiring companies with any reasonable market share will argue for their case, its textbook economics and what the FTC is trained for.
You’re getting downvoted because nobody gives a rats ass about efficiency for massive corporations that continue to abuse their power and make our lives unnecessarily expensive while their leaders (generally) tear down democracy.
Then I have failed to explain myself. What I was saying is that "economies of scope will reduce prices due to reduced costs" will be the core argument massive corps argue when merging and concentrating market size -- and that rarely happens in reality.