The commercial radio comparison is a good one. Commercial radio shows what happens when PSB regulations are diluted. We no longer have commercial radio stations serving a local audience and being active in the community, just pseudo-national jukeboxes. Not much variety either. Both major commercial broadcasters prefer to compete with each other within similar markets / genres rather than being unique / providing something different. That's what will happen to TV if the PSB requirements are diluted.
The D3&4 renewal suggestions are interesting as this multiplex is run by PSB's. I can't honestly see a fully commercial operator wanting to take over, especially not with the number of relays this multiplex operates.
Of course, that's another element of the PSB requirements, that the broadcasters have to reach most of the population. Purely commercial operators have no interest in reaching the last 10% - as we have seen with the COM multiplexes.