To use another analogy, imagine a market where an individual monopolizes 15% of stalls without selling any products, yet he still wants to paid.
Nope, he owns 15% of some flats but ONLY rents them to people who don't actually plan on living there. This way he gets his fixed income (50 BTC/block) but not the extra tips/gifts people give to their landlord at christmas or so (transaction fees). On the other hand he'll never have the hassle of having to fix broken pipes, deal with angy people in his flats etc.
Also to put it into perspective: it now takes 15% longer to get a (dead cheap) transaction in the system - 11.5 minutes instead of 10. If you wait for 6 blocks for confirmation, that's 90 seconds more to 1 hour of waiting, meaning 3690 instead of 3600 seconds. On the other hand it is 15% more difficult to attack the Bitcoin blockchain, which surely is also worth something!
Some solutions proposed here might make it even harder to scale bitcoin up to a relevant level (I call 1 transaction every ~5 seconds not really relevant...) and also will further disincentivize people from mining. There is already the drop to 25 coins per block in a bit less than 9 months which will make mining MUCH harder, if the price for Bitcoins doesn't double or the security (=difficulty) of the block chain doesn't get cut in half too because so many are quitting mining.
TX fees don't have to be raised at all - BUT if someone doesn't choose to pay a fair price (which TX fees currently really aren't), why should it even be forced upon miners to include more transactions that they feel comfortable with? Also this kind of stuff can lead to more branching of blocks, as it will (hopefully) only be a feature of the satoshi client.
Food for thought:
If someone already owns a botnet that can hash as fast as 15% of all miners together (if using CPU that's a LOT of computers!), don't you think it might also be possible to deploy a stripped down "dumb" client there, that relays blocks to as many satoshi clients as possible? The few ten thousand satoshi clients then would delay blocks between each other - but it wouldn't really matter, as they would nearly instantaneously anyways get the blocks from the "blockrelay" client.