The counterpoint to this is that one of the more profitable parts of Deliveroo's business is to do away with these local restaurants entirely in favour of ghost kitchens (which Deliver refers to as "Deliveroo Editions").
So Deliveroo enters a marketplace, sees which local restaurants are doing a brisk trade, and brings in a ghost kitchen that they own and operate (with the higher margins that cutting out an additional party entails).
And to the end user - yourself - this can be very hard to determine, and you can believe yourself to be supporting a local eatery when in fact you're not. In some regards this is not dissimilar to Amazon's approach (find out what third party products are selling well, release their own version and undercut).
Ghost kitchens are a good way for restaurant chains to expand by opening these types of kitchens for delivery only instead of having to open a full-blown restaurant at a premium.
For customers it does not matter that their Nando's (just an example, I don't know if Nando's do ghost kitchens) comes from a Nando's full restaurant or a Nando's ghost kitchen. It's exactly the same food and it creates the same number of jobs for people working in kitchens...
It's not Deliveroo that can decide that "Oh, Nando's do a good business here so let's create a fake one by ourselves...".
Now, on the other hand for independent restaurants Deliveroo (and other similar services obviously) can be a good way to expand their customers base by offering delivery without having to invest in the infrastructure.
Any restaurant that went through the effort of distinguishing themselves and specializing in a few signature dishes is unlikely to be threatened by ghost kitchens.
This is not too difficult to believe and to guess the mechanism at play: Before Deliveroo (and similar) services many local restaurants could only be reached by actually visiting them and eating them. Now they can reach people who want to eat at home (and who previously could only order from some pizza/kebab places that were the only ones to deliver). Therefore, all in all, I can believe that they made people order more than before and effectively consume more restaurant food than before. For my household that has certainly been the case.
[1] https://en.wikipedia.org/wiki/Deliveroo