I don't see that 'massive' is necessarily a reason for 'departmentalisation' in the sense we are talking about. The tax/NI systems, DVLA, TV licence (and even Covid-19 vaccination!) systems seem to have successfully avoided that 'need'. Indeed, the considerable advances in IT systems mean that it is now not really any more difficult to have billing all undertaken by a single 'system' than having it split in umpteen ones (although that would probably have been much more difficult a few decades ago).The industry has always been partitioned into generation, distribution and local electricity boards. CEGB and local electricity boards to whom you would pay your bills. The industry was so massive, it had to be departmentalised. The difference now, is simply that the local boards have been replaced by independent companies, which can bid for your business and you are free to choose any of them.
Don't forget that, as I just wrote, the only 'service' the 'suppliers' provide is billing (and installation of meters to facilitate that).
A fixed charge for administration of billing (by whoever) would be fair enough, but to have a 'percentage mark-up' of the cost of all energy used added on by a company that provides no service other than billing is, in my opinion, not reasonable. It costs little, if anything, more to issue a bill for 10,000 kWh of electricity than a bill for 10 kWh.
There is no such thing as a free dinner. "Cost have come down" primarily for those consumers have gambled that supplier's gambles for fixed-price contracts as regards future energy prices prove to be successful. If one loses that gamble, then the company may go bust - and I'm not sure how reasonable it should be that consumers in general should have to pick up the cost of that - in general, if one gambles and loses, that is one's own problem. With variable-price contracts, it's obviously the government's 'cap' which is causing most of the problem (for suppliers), but that is currently acting in a manner that was never intended/envisaged.As a result, costs have come down for the consumer. I doubt any of them are making big profits, as we have seen recently, many have been caught out and gone bust.
Kind Regards, John