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).

How would this be a monopoly?

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.