That's not what a rollup is.
A rollup is where you merge businesses doing functionally similar things, gaining economies of scale on costs. Its most often used for geographically distributed family businesses, such as Dentists. Dentists all need a lot of back office support, marketing, and sales functions. Having a rollup with a dozen Dental offices allows you to centralize all the insurance claim handling, direct marketing and put them under a single brand you can advertise on more mass market media.
You are correct that where rollups are used in regulated industries that limit entrants such as radio, they lead to oligopolies. If you own most of the radio stations in a single city you not only get the rollup cost benefits, you can also increase your prices for advertising.
But this is a direct effect of the market limitations that would occur either way, even if owners weren't allowed to reduce costs by buying stations in the same city to combine back office and operations, they would still do it to get oligarchic pricing power. And if you try to rollup most of the Dentist offices in a single city to raise prices you can only succeed temporarily, because it's a free market and your higher prices will attract lots of new entrants.
Of course the other criticisms of rollups is they reduce diversity of offerings. In the case of radio it leads to homogenization of station playlists. Which is exactly what the BBC does in Britain, so I don't see the issue. Especially since radio is being gutted by the internet anyways.