Potentially AFAIU, these rollup nodes can be required to be much more beefy than on L1, right? Why would anyone run a second rollup node? Only to try proving the deployer rollup node being wrong, and win 5k xtz? Is it up to the rollup deployer to code sufficient incentives for others to join with their rollup node?
You understood correctly. We believe it is in the rollup deployer's best interest to have other nodes participating and thus increasing trust. As it is also in the best interest of the community relying on a given rollup to have an independent third-party, running nodes to ensure everyone's funds are safe.
In the future, we hope to see businesses emerging around these community needs, in the form of “rollup as a service” providers. These providers will probably primarily be used by operators to manage their nodes, but they also could be used by third parties interested in tracking the state of a particular rollup. This is possible thanks to the open and decentralized nature of Smart Rollups.