Unless I missed something, outsourcing watching the blockchain cannot be called trustless; they need to know what to watch. If your watcher collaborates with your LN-hub, you are doomed.
Here is my get-rich-quick-scheme:
* Setup a big LN hub.
* Setup a LN-watching service
Then for each peer connected to both, I filter those that:
* have a balance with <10% on my side
* have an earlier balance with >90% on my side
* haven't been seen for a week.
For those, I submit the tx of the earlier balance. Sometimes I'll lose a bit but mostly I will gain more.
Profit.
(e.g. the large parked capital requirements acting as a barrier to entry, fees - if bitcoin is to be a low-volume settlement layer - plus people wanting to keep spending money consolidated (i.e. opening as few channels as possible) while still getting the benefit of complete payment connectivity)
Plus it won't work (in the sense of saving blockchain space) for people who have few peers and tend to move money in one direction, e.g. a consumer connected to a "paypal"-like lightning service who makes purchases but doesn't sell online, this could be a significant portion of users.
It's a great extension to Bitcoin, but it shouldn't be confused with what Bitcoin does.