Large landlords who own an entire street/area in upmarket or aspiring parts of a city also worry about low end tenants dragging the rest down. Completely rationally they'd rather have 60% occupancy at 2x the average rent than 95% at 1x and they know that high end retailers will pay a lot less (if anything) for a site next to a McDonald's (for example) than for a site next to another high end retailer.