Having a large dish gives you a large collecting area and hence better sensitivity. Building a multitude of receivers with the same collecting area, each having its own feed and electronics, is more expensive, not less. Otherwise that is what people would have done in the past. Arrays are built because you can synthesize a larger diameter of aperture. In addition, the noise properties of 100 receivers, each with 1/100 of the collecting area of a big dish do not give the same sensitivity, because you will essentially be getting additional noise for each receiver that is added.

Having a large dish gives you a large collecting area and hence better sensitivity. Building a multitude of receivers with the same collecting area, each having its own feed and electronics, is more expensive, not less. So yes, there is still a role for big dishes - at least on the scale of a few hundred metres. The fact that the Chinese have already built the 500-m diameter FAST radio telescope, rather than an array with similar collecting area, is evidence of that. 

Arrays are built chiefly because you can synthesize a larger diameter of aperture and get better angular resolution. In addition to cost, the noise properties of 100 receivers, each with 1/100 of the collecting area of a big dish would not give the same sensitivity, because you will essentially be getting additional noise for each receiver that is added.