Aside from the insane costs of opening a new restaurant, new locations aren't helpful if they are outside the 4 mile distance that most delivery services cap restaurants at. So there's that.

but I think it's on the restaurant to know what their capacity is and protect it

...that's why restaurants are dropping these services. They try putting caps in place with the services and the ordering services ignore it, or customers try to work around the caps. For example, most restaurants have a $300 order limit, but there's nothing to stop a customer from putting in two $300 orders for the same time. (Aka a $600 order.) Or some ordering apps don't let you order more than 24 hours in advance, which doesn't given the restaurant enough time to purchase extra supplies for big orders. The delivery website is just an ordering form-- it's not a restaurant manager. It can't think for itself. The restaurant usually has to confirm orders, and at that point it's almost too late to say no. If I, as a customer, spend 40 mins filling my cart with a huge order, and submit it, only to find out the restaurant can't handle the order, I'm going to be pissed at the restaurant. It makes the restaurant look bad. And sometimes the delivery service will disable a restaurant when they are too busy so folks can't order there, only that usually only happens with a restaurant is like "fuck off, we can't take orders anymore". And there are like 10 different delivery service websites now. You can't be updating all of them every hour based on how busy you are.

Sometimes the simplest solution is to drop all the services all together. And it's really fucked up when the services keep the restaurant on the site even when they are asked to be taken down.