1. Their website is really quite poor, and has been for years. Why the hell is the navigation so crap, and why can't I search for a specific food item?
2. They still haven't done anything about the prevalent and pervasive fake reviews.
Also, they cannot "move up-scale", their logo is associated with cheap food. Upscale restaurants don't want to put themselves on just-eat, next to the cheap low end places, nor do they want the just eat logo in their windows.
IMO it's the opposite of what you describe - it's way harder to go low-end to high-end. Just eat has to somehow convince people their brand isn't for bad food.and cheap takeaways, then market themselves at high-end places.
Deliveroo just needs to keep their high-end image, and then all the cheap places would throw themselves at Deliveroo to up their own image. No convincing required.