This is not the only consideration regarding centralisation, and not the main one IMO.
A crucial element is that many people can and will farm using spare resources that are already paid for in terms of both equipment and energy costs. This will be possible for the average Joe because Safecoin farming is based on proof of resource, whereas proof of work would favour those with the most powerful hardware.
The design is intended to ensure that there will be a lot of people farming with spare resources - just download the software, allocate some disk and bandwidth, then forget about it. These people can earn enough Safecoin to pay for their day to day use of SAFEnetwork to store their data (forever, as this is a one off cost), handle their SAFE email etc
Many will be profitable at a very low farming rate because their incremental costs are zero. Any Safecoin earned will be profit.
In turn, their numbers can be be a strong factor keeping the network average down, balancing the upward factor you identified.
So why is this decentralising? Because it keeps network rewards down. Why? Because farmers are in competition with each other for data. And this makes it harder to turn a profit with a business model that has to cover hardware and energy costs.
Rather than faster or bigger being better full stop - a "winner takes all" arms race to the top - there will probably be a sweet spot, somewhere above the network average where moderately sized dedicated farmers can turn a small profit. But the design is intended to ensure this is limited - too many vaults or too big makes too much competition, the rewards will fall and their profits won't cover costs.
Simply having more storage, bandwidth or CPU does not make you more profitable - you also have to earn enough to pay for that - while competing with very larger numbers of people farming on the machines and devices they are using for other things, as and when they're switched on, rather than as dedicated farming rigs.
This is a big difference from bitcoin, and something I've seen people struggle to get because of that.