Increasing shade would reduce bloom and produce thinner growth, but would not kill the azaleas all by itself. As the maples grow, however, their roots can out-compete the azaleas for nutrients and water. The same thing would happen to just about anything, including hostas, you might plant in this area. Thinning out the maple canopy, paying close attention to watering and mulching and providing extra fertilizer will help the azaleas, assuming there is no other cause for their decline. Rescuing what you have, with their established root systems, is likely to be a much better option than planting something new.