And PCs - it started out as a decentralized market between Apple, Commodore, Tandy, Atari, Osborne, etc, but then centralized under IBM before being decentralized again with IBM clones like Dell, Compaq, etc. That in turn forced the centralization of the OS market under Microsoft (driving out Apple, CP/M, Acorn, Atari) and microprocessors under Intel (driving out Motorola and MOS technologies).
I think it's more accurate to say that the point of centralization moves up and down the value chain based on technological developments, market deals, and corporate strategy. When things get too centralized, a savvy entrepreneur will come in as a small-time complement, then find a way to encourage competition and commoditize his complements, riding the wave to a very prosperous monopoly. When competition becomes too intense in those markets, the bigger players will start eating the smaller players, and eventually the market consolidates.
Ask an economist and you'll get different normative answers of how we should craft policy, but the positive answer will be the same across the board: it has happened with clothing manufacturing, publishing, radio, TV, food production, etc. and is now happening with the internet. A nascent and initially fragmented medium/industry attracts investment and develops standards; large conglomerations begin to form and M&A smaller players to gain market share, leading to a saturated market of heavy-weight players.