I trust S3, B2 Google's blobstore more than some rando's machine who runs filecoin. Tahoe-LAFS gives you the assurance that the actual backend storage only sees encrypted data. The big clouds have this advantage that they are probably more reliable, faster, have lower latency, better uptime, and lower price.
The Tahoe LAFS model is more that you spread your data over multiple providers. NAS at home, several VPS providers, or what have you. It feels like RAID in the network, and it allows very precise setting of redundancy policies.
In the PrivateStorage case the machines aren't "trusted" but they are all run by the service you're paying for -- so the incentive to keep them running properly is indeed there.
For other kinds of Tahoe deployments, no there's nothing built-in to incentivize storage-server operators. That part is up to whomever is organizing and running the Grid (what Tahoe calls a group of storage-servers). For example, friends could agree to host storage-servers for each other and create redundancy + trust that way.
The difference between Tahoe and things like Storj / FileCoin is that those services intend to be "a single, global service" whereas Tahoe is software that can be deployed in several different ways -- one of which is a professionally managed Grid such as PrivateStorage.
If you are interested in these topics I'd encourage you to join #tahoe-lafs on Freenode or one of the Tahoe development meetings. These are definitely things I've seen discussed but I think Tahoe-LAFS is far more likely to introduce a concept of "federated Grids" rather than "a single global Tahoe service".
Sounds risky.
Do storage providers have an incentive to provide the service reliably à la filecoin? [0]
[0] https://filecoin.io/